On Thu, Jun 15, 2017 at 12:06:43AM +0200, Erik Skultety wrote: > Hi all, > > so there's been an off-list discussion about finally implementing creation of > mediated devices with libvirt and it's more than desired to get as many opinions > on that as possible, so please do share your ideas. This did come up already as > part of some older threads ([1] for example), so this will be a respin of the > discussions. Long story short, we decided to put device creation off and focus > on the introduction of the framework as such first and build upon that later, > i.e. now. > > [1] https://www.redhat.com/archives/libvir-list/2017-February/msg00177.html > > ======================================== > PART 1: NODEDEV-DRIVER > ======================================== > > API-wise, device creation through the nodedev driver should be pretty > straightforward and without any issues, since virNodeDevCreateXML takes an XML > and does support flags. Looking at the current device XML: > > <device> > <name>mdev_0cce8709_0640_46ef_bd14_962c7f73cc6f</name> > <path>/sys/devices/pci0000:00/.../0cce8709-0640-46ef-bd14-962c7f73cc6f</path> > <parent>pci_0000_03_00_0</parent> > <driver> > <name>vfio_mdev</name> > </driver> > <capability type='mdev'> > <type id='nvidia-11'/> > <iommuGroup number='13'/> > <uuid>UUID<uuid> <!-- optional enhancement, see below --> > </capability> > </device> > > We can ignore <path>,<driver>,<iommugroup> elements, since these are useless > during creation. We also cannot use <name> since we don't support arbitrary > names and we also can't rely on users providing a name in correct form which we > would need to further parse in order to get the UUID. > So since the only thing missing to successfully use create an mdev using XML is > the UUID (if user doesn't want it to be generated automatically), how about > having a <uuid> subelement under <capability> just like PCIs have <domain> and > friends, USBs have <bus> & <device>, interfaces have <address> to uniquely > identify the device even if the name itself is unique. > Removal of a device should work as well, although we might want to > consider creating a *Flags version of the API. > > ============================================================= > PART 2: DOMAIN XML & DEVICE AUTO-CREATION, NO POLICY INVOLVED! > ============================================================= > > There were some doubts about auto-creation mentioned in [1], although they > weren't specified further. So hopefully, we'll get further in the discussion > this time. > > From my perspective there are two main reasons/benefits to that: > > 1) Convenience > For apps like virt-manager, user will want to add a host device transparently, > "hey libvirt, I want an mdev assigned to my VM, can you do that". Even for > higher management apps, like oVirt, even they might not care about the parent > device at all times and considering that they would need to enumerate the > parents, pick one, create the device XML and pass it to the nodedev driver, IMHO > it would actually be easier and faster to just do it directly through sysfs, > bypassing libvirt once again.... The convenience only works if the policy we've provided in libvirt actually matches the policy the application wants. I think it is quite likely that with cloud the mdevs will be created out of band from the domain startup process. It is possible the app will just have a fixed set of mdevs pre-created when the host starts up. Or that the mgmt app wants the domain startup process to be a two phase setup, where it first allocates the resources needed, and later then tries to start the guest. This is why I keep saying that putting this kind of "convenient" policy in libvirt is a bad idea - it is essentially just putting a bit of virt-manager code into libvirt - more advanced apps will need more flexibility in this area. > 2) Future domain migration > Suppose now that the mdev backing physical devices support state dump and > reload. Chances are, that the corresponding mdev doesn't even exist or has a > different UUID on the destination, so libvirt would do its best to handle this > before the domain could be resumed. This is not an unusual scenario - there are already many other parts of the device backend config that need to change prior to migration, especially for anything related to host devices, so apps already have support for doing this, which is more flexible & convenient becasue it doesn't tie creation of the mdevs to running of the migrate command. IOW, I'm still against adding any kind of automatic creation policy for mdevs in libvirt. Just provide the node device API support.

On 06/15/2017 02:42 PM, Alex Williamson wrote: > On Thu, 15 Jun 2017 09:33:01 +0100 > "Daniel P. Berrange" <berrange(a)redhat.com&gt; wrote: > >> On Thu, Jun 15, 2017 at 12:06:43AM +0200, Erik Skultety wrote: >>> Hi all, >>> >>> so there's been an off-list discussion about finally implementing creation of >>> mediated devices with libvirt and it's more than desired to get as many opinions >>> on that as possible, so please do share your ideas. This did come up already as >>> part of some older threads ([1] for example), so this will be a respin of the >>> discussions. Long story short, we decided to put device creation off and focus >>> on the introduction of the framework as such first and build upon that later, >>> i.e. now. >>> >>> [1] https://www.redhat.com/archives/libvir-list/2017-February/msg00177.html >>> >>> ======================================== >>> PART 1: NODEDEV-DRIVER >>> ======================================== >>> >>> API-wise, device creation through the nodedev driver should be pretty >>> straightforward and without any issues, since virNodeDevCreateXML takes an XML >>> and does support flags. Looking at the current device XML: >>> >>> <device> >>> <name>mdev_0cce8709_0640_46ef_bd14_962c7f73cc6f</name> >>> <path>/sys/devices/pci0000:00/.../0cce8709-0640-46ef-bd14-962c7f73cc6f</path> >>> <parent>pci_0000_03_00_0</parent> >>> <driver> >>> <name>vfio_mdev</name> >>> </driver> >>> <capability type='mdev'> >>> <type id='nvidia-11'/> >>> <iommuGroup number='13'/> >>> <uuid>UUID<uuid> <!-- optional enhancement, see below --> >>> </capability> >>> </device> >>> >>> We can ignore <path>,<driver>,<iommugroup> elements, since these are useless >>> during creation. We also cannot use <name> since we don't support arbitrary >>> names and we also can't rely on users providing a name in correct form which we >>> would need to further parse in order to get the UUID. >>> So since the only thing missing to successfully use create an mdev using XML is >>> the UUID (if user doesn't want it to be generated automatically), how about >>> having a <uuid> subelement under <capability> just like PCIs have <domain> and >>> friends, USBs have <bus> & <device>, interfaces have <address> to uniquely >>> identify the device even if the name itself is unique. >>> Removal of a device should work as well, although we might want to >>> consider creating a *Flags version of the API. >>> >>> ============================================================= >>> PART 2: DOMAIN XML & DEVICE AUTO-CREATION, NO POLICY INVOLVED! >>> ============================================================= >>> >>> There were some doubts about auto-creation mentioned in [1], although they >>> weren't specified further. So hopefully, we'll get further in the discussion >>> this time. >>> >>> From my perspective there are two main reasons/benefits to that: >>> >>> 1) Convenience >>> For apps like virt-manager, user will want to add a host device transparently, >>> "hey libvirt, I want an mdev assigned to my VM, can you do that". Even for >>> higher management apps, like oVirt, even they might not care about the parent >>> device at all times and considering that they would need to enumerate the >>> parents, pick one, create the device XML and pass it to the nodedev driver, IMHO >>> it would actually be easier and faster to just do it directly through sysfs, >>> bypassing libvirt once again.... >> >> The convenience only works if the policy we've provided in libvirt actually >> matches the policy the application wants. I think it is quite likely that with >> cloud the mdevs will be created out of band from the domain startup process. >> It is possible the app will just have a fixed set of mdevs pre-created when >> the host starts up. Or that the mgmt app wants the domain startup process to >> be a two phase setup, where it first allocates the resources needed, and later >> then tries to start the guest. This is why I keep saying that putting this kind >> of "convenient" policy in libvirt is a bad idea - it is essentially just putting >> a bit of virt-manager code into libvirt - more advanced apps will need more >> flexibility in this area. >> >>> 2) Future domain migration >>> Suppose now that the mdev backing physical devices support state dump and >>> reload. Chances are, that the corresponding mdev doesn't even exist or has a >>> different UUID on the destination, so libvirt would do its best to handle this >>> before the domain could be resumed. >> >> This is not an unusual scenario - there are already many other parts of the >> device backend config that need to change prior to migration, especially for >> anything related to host devices, so apps already have support for doing >> this, which is more flexible & convenient becasue it doesn't tie creation of >> the mdevs to running of the migrate command. >> >> IOW, I'm still against adding any kind of automatic creation policy for >> mdevs in libvirt. Just provide the node device API support. > > I'm not super clear on the extent of what you're against here, is it > all forms of device creation or only a placement policy? Are you > against any form of having the XML specify the non-instantiated mdev > that it wants? We've clearly made an important step with libvirt > supporting pre-created mdevs, but as a user of that support I find it > incredibly tedious. I typically do a dumpxml, copy out the UUID, > wonder what type of device it might have been last time, create it, > start the domain and cross my fingers. Pre-creating mdev devices is not > really practical, I might have use cases where I want multiple low-end > mdev devices and another where I have a single high-end device. Those > cannot exist at the same time. Requiring extensive higher level > management tools is not really an option either, I'm not going to > install oVirt on my desktop/laptop just so I can launch a GVT-g VM once > in a while (no offense). So I really hope that libvirt itself can > provide some degree of mdev creation. Maybe there can be something in between the "all child devices must be pre-created" and "a child device will be automatically created on an automatically chosen parent device as needed". In particular, we could forego the "automatically chosen parent device" part of that. The guest configuration could simply contain the PCI address of the parent and the desired type of the child. If we did this there wouldn't be any policy decision to make - all the variables are determined - but it would make life easier for people running small hosts (i.e. no oVirt/Openstack, a single mdev parent device). Openstack and oVirt (and whoever) would of course be free to ignore this and pre-create pools of devices themselves in the name of more precise control and better predictability (just as, for example, OpenStack ignores libvirt's "pools of hostdev network devices" and instead manages the pool of devices itself and uses <interface type='hostdev'> directly).

On Fri, 16 Jun 2017 11:32:04 -0400 Laine Stump <laine(a)redhat.com&gt; wrote: > On 06/15/2017 02:42 PM, Alex Williamson wrote: > > On Thu, 15 Jun 2017 09:33:01 +0100 > > "Daniel P. Berrange" <berrange(a)redhat.com&gt; wrote: > > > >> On Thu, Jun 15, 2017 at 12:06:43AM +0200, Erik Skultety wrote: > >>> Hi all, > >>> > >>> so there's been an off-list discussion about finally implementing creation of > >>> mediated devices with libvirt and it's more than desired to get as many opinions > >>> on that as possible, so please do share your ideas. This did come up already as > >>> part of some older threads ([1] for example), so this will be a respin of the > >>> discussions. Long story short, we decided to put device creation off and focus > >>> on the introduction of the framework as such first and build upon that later, > >>> i.e. now. > >>> > >>> [1] https://www.redhat.com/archives/libvir-list/2017-February/msg00177.html > >>> > >>> ======================================== > >>> PART 1: NODEDEV-DRIVER > >>> ======================================== > >>> > >>> API-wise, device creation through the nodedev driver should be pretty > >>> straightforward and without any issues, since virNodeDevCreateXML takes an XML > >>> and does support flags. Looking at the current device XML: > >>> > >>> <device> > >>> <name>mdev_0cce8709_0640_46ef_bd14_962c7f73cc6f</name> > >>> <path>/sys/devices/pci0000:00/.../0cce8709-0640-46ef-bd14-962c7f73cc6f</path> > >>> <parent>pci_0000_03_00_0</parent> > >>> <driver> > >>> <name>vfio_mdev</name> > >>> </driver> > >>> <capability type='mdev'> > >>> <type id='nvidia-11'/> > >>> <iommuGroup number='13'/> > >>> <uuid>UUID<uuid> <!-- optional enhancement, see below --> > >>> </capability> > >>> </device> > >>> > >>> We can ignore <path>,<driver>,<iommugroup> elements, since these are useless > >>> during creation. We also cannot use <name> since we don't support arbitrary > >>> names and we also can't rely on users providing a name in correct form which we > >>> would need to further parse in order to get the UUID. > >>> So since the only thing missing to successfully use create an mdev using XML is > >>> the UUID (if user doesn't want it to be generated automatically), how about > >>> having a <uuid> subelement under <capability> just like PCIs have <domain> and > >>> friends, USBs have <bus> & <device>, interfaces have <address> to uniquely > >>> identify the device even if the name itself is unique. > >>> Removal of a device should work as well, although we might want to > >>> consider creating a *Flags version of the API. > >>> > >>> ============================================================= > >>> PART 2: DOMAIN XML & DEVICE AUTO-CREATION, NO POLICY INVOLVED! > >>> ============================================================= > >>> > >>> There were some doubts about auto-creation mentioned in [1], although they > >>> weren't specified further. So hopefully, we'll get further in the discussion > >>> this time. > >>> > >>> From my perspective there are two main reasons/benefits to that: > >>> > >>> 1) Convenience > >>> For apps like virt-manager, user will want to add a host device transparently, > >>> "hey libvirt, I want an mdev assigned to my VM, can you do that". Even for > >>> higher management apps, like oVirt, even they might not care about the parent > >>> device at all times and considering that they would need to enumerate the > >>> parents, pick one, create the device XML and pass it to the nodedev driver, IMHO > >>> it would actually be easier and faster to just do it directly through sysfs, > >>> bypassing libvirt once again.... > >> > >> The convenience only works if the policy we've provided in libvirt actually > >> matches the policy the application wants. I think it is quite likely that with > >> cloud the mdevs will be created out of band from the domain startup process. > >> It is possible the app will just have a fixed set of mdevs pre-created when > >> the host starts up. Or that the mgmt app wants the domain startup process to > >> be a two phase setup, where it first allocates the resources needed, and later > >> then tries to start the guest. This is why I keep saying that putting this kind > >> of "convenient" policy in libvirt is a bad idea - it is essentially just putting > >> a bit of virt-manager code into libvirt - more advanced apps will need more > >> flexibility in this area. > >> > >>> 2) Future domain migration > >>> Suppose now that the mdev backing physical devices support state dump and > >>> reload. Chances are, that the corresponding mdev doesn't even exist or has a > >>> different UUID on the destination, so libvirt would do its best to handle this > >>> before the domain could be resumed. > >> > >> This is not an unusual scenario - there are already many other parts of the > >> device backend config that need to change prior to migration, especially for > >> anything related to host devices, so apps already have support for doing > >> this, which is more flexible & convenient becasue it doesn't tie creation of > >> the mdevs to running of the migrate command. > >> > >> IOW, I'm still against adding any kind of automatic creation policy for > >> mdevs in libvirt. Just provide the node device API support. > > > > I'm not super clear on the extent of what you're against here, is it > > all forms of device creation or only a placement policy? Are you > > against any form of having the XML specify the non-instantiated mdev > > that it wants? We've clearly made an important step with libvirt > > supporting pre-created mdevs, but as a user of that support I find it > > incredibly tedious. I typically do a dumpxml, copy out the UUID, > > wonder what type of device it might have been last time, create it, > > start the domain and cross my fingers. Pre-creating mdev devices is not > > really practical, I might have use cases where I want multiple low-end > > mdev devices and another where I have a single high-end device. Those > > cannot exist at the same time. Requiring extensive higher level > > management tools is not really an option either, I'm not going to > > install oVirt on my desktop/laptop just so I can launch a GVT-g VM once > > in a while (no offense). So I really hope that libvirt itself can > > provide some degree of mdev creation. > > > Maybe there can be something in between the "all child devices must be > pre-created" and "a child device will be automatically created on an > automatically chosen parent device as needed". In particular, we could > forego the "automatically chosen parent device" part of that. The guest > configuration could simply contain the PCI address of the parent and the > desired type of the child. If we did this there wouldn't be any policy > decision to make - all the variables are determined - but it would make > life easier for people running small hosts (i.e. no oVirt/Openstack, a > single mdev parent device). Openstack and oVirt (and whoever) would of > course be free to ignore this and pre-create pools of devices themselves > in the name of more precise control and better predictability (just as, > for example, OpenStack ignores libvirt's "pools of hostdev network > devices" and instead manages the pool of devices itself and uses > <interface type='hostdev'> directly). This seems not that substantially different from managed='yes' on a vfio hostdev to me. It makes the device available to the VM before it starts and returns it after. In one case that's switching the binding on an existing device, in another it's creating and removing. Once again, I can't tell from Dan's response if he's opposed to this entire idea or just the aspects where libvirt needs to impose a policy decision. For me personally, the functionality difference is quite substantial.

On Fri, Jun 16, 2017 at 06:11:17PM +0100, Daniel P. Berrange wrote: > On Fri, Jun 16, 2017 at 11:02:55AM -0600, Alex Williamson wrote: > > On Fri, 16 Jun 2017 11:32:04 -0400 > > Laine Stump <laine(a)redhat.com&gt; wrote: > > > > > On 06/15/2017 02:42 PM, Alex Williamson wrote: > > > > On Thu, 15 Jun 2017 09:33:01 +0100 > > > > "Daniel P. Berrange" <berrange(a)redhat.com&gt; wrote: > > > > > > > >> On Thu, Jun 15, 2017 at 12:06:43AM +0200, Erik Skultety wrote: > > > >>> Hi all, > > > >>> > > > >>> so there's been an off-list discussion about finally implementing creation of > > > >>> mediated devices with libvirt and it's more than desired to get as many opinions > > > >>> on that as possible, so please do share your ideas. This did come up already as > > > >>> part of some older threads ([1] for example), so this will be a respin of the > > > >>> discussions. Long story short, we decided to put device creation off and focus > > > >>> on the introduction of the framework as such first and build upon that later, > > > >>> i.e. now. > > > >>> > > > >>> [1] https://www.redhat.com/archives/libvir-list/2017-February/msg00177.html > > > >>> > > > >>> ======================================== > > > >>> PART 1: NODEDEV-DRIVER > > > >>> ======================================== > > > >>> > > > >>> API-wise, device creation through the nodedev driver should be pretty > > > >>> straightforward and without any issues, since virNodeDevCreateXML takes an XML > > > >>> and does support flags. Looking at the current device XML: > > > >>> > > > >>> <device> > > > >>> <name>mdev_0cce8709_0640_46ef_bd14_962c7f73cc6f</name> > > > >>> <path>/sys/devices/pci0000:00/.../0cce8709-0640-46ef-bd14-962c7f73cc6f</path> > > > >>> <parent>pci_0000_03_00_0</parent> > > > >>> <driver> > > > >>> <name>vfio_mdev</name> > > > >>> </driver> > > > >>> <capability type='mdev'> > > > >>> <type id='nvidia-11'/> > > > >>> <iommuGroup number='13'/> > > > >>> <uuid>UUID<uuid> <!-- optional enhancement, see below --> > > > >>> </capability> > > > >>> </device> > > > >>> > > > >>> We can ignore <path>,<driver>,<iommugroup> elements, since these are useless > > > >>> during creation. We also cannot use <name> since we don't support arbitrary > > > >>> names and we also can't rely on users providing a name in correct form which we > > > >>> would need to further parse in order to get the UUID. > > > >>> So since the only thing missing to successfully use create an mdev using XML is > > > >>> the UUID (if user doesn't want it to be generated automatically), how about > > > >>> having a <uuid> subelement under <capability> just like PCIs have <domain> and > > > >>> friends, USBs have <bus> & <device>, interfaces have <address> to uniquely > > > >>> identify the device even if the name itself is unique. > > > >>> Removal of a device should work as well, although we might want to > > > >>> consider creating a *Flags version of the API. > > > >>> > > > >>> ============================================================= > > > >>> PART 2: DOMAIN XML & DEVICE AUTO-CREATION, NO POLICY INVOLVED! > > > >>> ============================================================= > > > >>> > > > >>> There were some doubts about auto-creation mentioned in [1], although they > > > >>> weren't specified further. So hopefully, we'll get further in the discussion > > > >>> this time. > > > >>> > > > >>> From my perspective there are two main reasons/benefits to that: > > > >>> > > > >>> 1) Convenience > > > >>> For apps like virt-manager, user will want to add a host device transparently, > > > >>> "hey libvirt, I want an mdev assigned to my VM, can you do that". Even for > > > >>> higher management apps, like oVirt, even they might not care about the parent > > > >>> device at all times and considering that they would need to enumerate the > > > >>> parents, pick one, create the device XML and pass it to the nodedev driver, IMHO > > > >>> it would actually be easier and faster to just do it directly through sysfs, > > > >>> bypassing libvirt once again.... > > > >> > > > >> The convenience only works if the policy we've provided in libvirt actually > > > >> matches the policy the application wants. I think it is quite likely that with > > > >> cloud the mdevs will be created out of band from the domain startup process. > > > >> It is possible the app will just have a fixed set of mdevs pre-created when > > > >> the host starts up. Or that the mgmt app wants the domain startup process to > > > >> be a two phase setup, where it first allocates the resources needed, and later > > > >> then tries to start the guest. This is why I keep saying that putting this kind > > > >> of "convenient" policy in libvirt is a bad idea - it is essentially just putting > > > >> a bit of virt-manager code into libvirt - more advanced apps will need more > > > >> flexibility in this area. > > > >> > > > >>> 2) Future domain migration > > > >>> Suppose now that the mdev backing physical devices support state dump and > > > >>> reload. Chances are, that the corresponding mdev doesn't even exist or has a > > > >>> different UUID on the destination, so libvirt would do its best to handle this > > > >>> before the domain could be resumed. > > > >> > > > >> This is not an unusual scenario - there are already many other parts of the > > > >> device backend config that need to change prior to migration, especially for > > > >> anything related to host devices, so apps already have support for doing > > > >> this, which is more flexible & convenient becasue it doesn't tie creation of > > > >> the mdevs to running of the migrate command. > > > >> > > > >> IOW, I'm still against adding any kind of automatic creation policy for > > > >> mdevs in libvirt. Just provide the node device API support. > > > > > > > > I'm not super clear on the extent of what you're against here, is it > > > > all forms of device creation or only a placement policy? Are you > > > > against any form of having the XML specify the non-instantiated mdev > > > > that it wants? We've clearly made an important step with libvirt > > > > supporting pre-created mdevs, but as a user of that support I find it > > > > incredibly tedious. I typically do a dumpxml, copy out the UUID, > > > > wonder what type of device it might have been last time, create it, > > > > start the domain and cross my fingers. Pre-creating mdev devices is not > > > > really practical, I might have use cases where I want multiple low-end > > > > mdev devices and another where I have a single high-end device. Those > > > > cannot exist at the same time. Requiring extensive higher level > > > > management tools is not really an option either, I'm not going to > > > > install oVirt on my desktop/laptop just so I can launch a GVT-g VM once > > > > in a while (no offense). So I really hope that libvirt itself can > > > > provide some degree of mdev creation. > > > > > > > > > Maybe there can be something in between the "all child devices must be > > > pre-created" and "a child device will be automatically created on an > > > automatically chosen parent device as needed". In particular, we could > > > forego the "automatically chosen parent device" part of that. The guest > > > configuration could simply contain the PCI address of the parent and the > > > desired type of the child. If we did this there wouldn't be any policy > > > decision to make - all the variables are determined - but it would make > > > life easier for people running small hosts (i.e. no oVirt/Openstack, a > > > single mdev parent device). Openstack and oVirt (and whoever) would of > > > course be free to ignore this and pre-create pools of devices themselves > > > in the name of more precise control and better predictability (just as, > > > for example, OpenStack ignores libvirt's "pools of hostdev network > > > devices" and instead manages the pool of devices itself and uses > > > <interface type='hostdev'> directly). > > > > This seems not that substantially different from managed='yes' on a > > vfio hostdev to me. It makes the device available to the VM before it > > starts and returns it after. In one case that's switching the binding > > on an existing device, in another it's creating and removing. Once > > again, I can't tell from Dan's response if he's opposed to this entire > > idea or just the aspects where libvirt needs to impose a policy > > decision. For me personally, the functionality difference is quite > > substantial. > > I'm fine with libvirt having APIs in the node device APIs to enable > create/delete with libvirt, as well as using managed=yes in the same > manner that we do for regular PCI devices (the bind/unbind to vfio > or pci-back) Oh, and we really need to fix the big missing feature in the node device APIs of persistent, inactive configs. eg we should be able to record XML configs of mdevs (and npiv devices too), in /etc/libvirt so they persist across reboots, and can be setup for auto-start on boot too.

Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|

On Thu, Jun 22, 2017 at 10:41:13AM +0200, Martin Polednik wrote: > On 16/06/17 18:14 +0100, Daniel P. Berrange wrote: > > On Fri, Jun 16, 2017 at 06:11:17PM +0100, Daniel P. Berrange wrote: > > > On Fri, Jun 16, 2017 at 11:02:55AM -0600, Alex Williamson wrote: > > > > On Fri, 16 Jun 2017 11:32:04 -0400 > > > > Laine Stump <laine(a)redhat.com&gt; wrote: > > > > > > > > > On 06/15/2017 02:42 PM, Alex Williamson wrote: > > > > > > On Thu, 15 Jun 2017 09:33:01 +0100 > > > > > > "Daniel P. Berrange" <berrange(a)redhat.com&gt; wrote: > > > > > > > > > > > >> On Thu, Jun 15, 2017 at 12:06:43AM +0200, Erik Skultety wrote: > > > > > >>> Hi all, > > > > > >>> > > > > > >>> so there's been an off-list discussion about finally implementing creation of > > > > > >>> mediated devices with libvirt and it's more than desired to get as many opinions > > > > > >>> on that as possible, so please do share your ideas. This did come up already as > > > > > >>> part of some older threads ([1] for example), so this will be a respin of the > > > > > >>> discussions. Long story short, we decided to put device creation off and focus > > > > > >>> on the introduction of the framework as such first and build upon that later, > > > > > >>> i.e. now. > > > > > >>> > > > > > >>> [1] https://www.redhat.com/archives/libvir-list/2017-February/msg00177.html > > > > > >>> > > > > > >>> ======================================== > > > > > >>> PART 1: NODEDEV-DRIVER > > > > > >>> ======================================== > > > > > >>> > > > > > >>> API-wise, device creation through the nodedev driver should be pretty > > > > > >>> straightforward and without any issues, since virNodeDevCreateXML takes an XML > > > > > >>> and does support flags. Looking at the current device XML: > > > > > >>> > > > > > >>> <device> > > > > > >>> <name>mdev_0cce8709_0640_46ef_bd14_962c7f73cc6f</name> > > > > > >>> <path>/sys/devices/pci0000:00/.../0cce8709-0640-46ef-bd14-962c7f73cc6f</path> > > > > > >>> <parent>pci_0000_03_00_0</parent> > > > > > >>> <driver> > > > > > >>> <name>vfio_mdev</name> > > > > > >>> </driver> > > > > > >>> <capability type='mdev'> > > > > > >>> <type id='nvidia-11'/> > > > > > >>> <iommuGroup number='13'/> > > > > > >>> <uuid>UUID<uuid> <!-- optional enhancement, see below --> > > > > > >>> </capability> > > > > > >>> </device> > > > > > >>> > > > > > >>> We can ignore <path>,<driver>,<iommugroup> elements, since these are useless > > > > > >>> during creation. We also cannot use <name> since we don't support arbitrary > > > > > >>> names and we also can't rely on users providing a name in correct form which we > > > > > >>> would need to further parse in order to get the UUID. > > > > > >>> So since the only thing missing to successfully use create an mdev using XML is > > > > > >>> the UUID (if user doesn't want it to be generated automatically), how about > > > > > >>> having a <uuid> subelement under <capability> just like PCIs have <domain> and > > > > > >>> friends, USBs have <bus> & <device>, interfaces have <address> to uniquely > > > > > >>> identify the device even if the name itself is unique. > > > > > >>> Removal of a device should work as well, although we might want to > > > > > >>> consider creating a *Flags version of the API. > > > > > >>> > > > > > >>> ============================================================= > > > > > >>> PART 2: DOMAIN XML & DEVICE AUTO-CREATION, NO POLICY INVOLVED! > > > > > >>> ============================================================= > > > > > >>> > > > > > >>> There were some doubts about auto-creation mentioned in [1], although they > > > > > >>> weren't specified further. So hopefully, we'll get further in the discussion > > > > > >>> this time. > > > > > >>> > > > > > >>> From my perspective there are two main reasons/benefits to that: > > > > > >>> > > > > > >>> 1) Convenience > > > > > >>> For apps like virt-manager, user will want to add a host device transparently, > > > > > >>> "hey libvirt, I want an mdev assigned to my VM, can you do that". Even for > > > > > >>> higher management apps, like oVirt, even they might not care about the parent > > > > > >>> device at all times and considering that they would need to enumerate the > > > > > >>> parents, pick one, create the device XML and pass it to the nodedev driver, IMHO > > > > > >>> it would actually be easier and faster to just do it directly through sysfs, > > > > > >>> bypassing libvirt once again.... > > > > > >> > > > > > >> The convenience only works if the policy we've provided in libvirt actually > > > > > >> matches the policy the application wants. I think it is quite likely that with > > > > > >> cloud the mdevs will be created out of band from the domain startup process. > > > > > >> It is possible the app will just have a fixed set of mdevs pre-created when > > > > > >> the host starts up. Or that the mgmt app wants the domain startup process to > > > > > >> be a two phase setup, where it first allocates the resources needed, and later > > > > > >> then tries to start the guest. This is why I keep saying that putting this kind > > > > > >> of "convenient" policy in libvirt is a bad idea - it is essentially just putting > > > > > >> a bit of virt-manager code into libvirt - more advanced apps will need more > > > > > >> flexibility in this area. > > > > > >> > > > > > >>> 2) Future domain migration > > > > > >>> Suppose now that the mdev backing physical devices support state dump and > > > > > >>> reload. Chances are, that the corresponding mdev doesn't even exist or has a > > > > > >>> different UUID on the destination, so libvirt would do its best to handle this > > > > > >>> before the domain could be resumed. > > > > > >> > > > > > >> This is not an unusual scenario - there are already many other parts of the > > > > > >> device backend config that need to change prior to migration, especially for > > > > > >> anything related to host devices, so apps already have support for doing > > > > > >> this, which is more flexible & convenient becasue it doesn't tie creation of > > > > > >> the mdevs to running of the migrate command. > > > > > >> > > > > > >> IOW, I'm still against adding any kind of automatic creation policy for > > > > > >> mdevs in libvirt. Just provide the node device API support. > > > > > > > > > > > > I'm not super clear on the extent of what you're against here, is it > > > > > > all forms of device creation or only a placement policy? Are you > > > > > > against any form of having the XML specify the non-instantiated mdev > > > > > > that it wants? We've clearly made an important step with libvirt > > > > > > supporting pre-created mdevs, but as a user of that support I find it > > > > > > incredibly tedious. I typically do a dumpxml, copy out the UUID, > > > > > > wonder what type of device it might have been last time, create it, > > > > > > start the domain and cross my fingers. Pre-creating mdev devices is not > > > > > > really practical, I might have use cases where I want multiple low-end > > > > > > mdev devices and another where I have a single high-end device. Those > > > > > > cannot exist at the same time. Requiring extensive higher level > > > > > > management tools is not really an option either, I'm not going to > > > > > > install oVirt on my desktop/laptop just so I can launch a GVT-g VM once > > > > > > in a while (no offense). So I really hope that libvirt itself can > > > > > > provide some degree of mdev creation. > > > > > > > > > > > > > > > Maybe there can be something in between the "all child devices must be > > > > > pre-created" and "a child device will be automatically created on an > > > > > automatically chosen parent device as needed". In particular, we could > > > > > forego the "automatically chosen parent device" part of that. The guest > > > > > configuration could simply contain the PCI address of the parent and the > > > > > desired type of the child. If we did this there wouldn't be any policy > > > > > decision to make - all the variables are determined - but it would make > > > > > life easier for people running small hosts (i.e. no oVirt/Openstack, a > > > > > single mdev parent device). Openstack and oVirt (and whoever) would of > > > > > course be free to ignore this and pre-create pools of devices themselves > > > > > in the name of more precise control and better predictability (just as, > > > > > for example, OpenStack ignores libvirt's "pools of hostdev network > > > > > devices" and instead manages the pool of devices itself and uses > > > > > <interface type='hostdev'> directly). > > > > > > > > This seems not that substantially different from managed='yes' on a > > > > vfio hostdev to me. It makes the device available to the VM before it > > > > starts and returns it after. In one case that's switching the binding > > > > on an existing device, in another it's creating and removing. Once > > > > again, I can't tell from Dan's response if he's opposed to this entire > > > > idea or just the aspects where libvirt needs to impose a policy > > > > decision. For me personally, the functionality difference is quite > > > > substantial. > > > > > > I'm fine with libvirt having APIs in the node device APIs to enable > > > create/delete with libvirt, as well as using managed=yes in the same > > > manner that we do for regular PCI devices (the bind/unbind to vfio > > > or pci-back) > > > > Oh, and we really need to fix the big missing feature in the node > > device APIs of persistent, inactive configs. eg we should be able > > to record XML configs of mdevs (and npiv devices too), in /etc/libvirt > > so they persist across reboots, and can be setup for auto-start on > > boot too. > > That doesn't help mdev in any way though. It doesn't make sense to > generate new UUID for given VM at each start. So in case of What statement does this^^ refer to? Why would you generate a new UUID for a VM at each start, you'd generate it only once and then store it, the same way as domain UUIDs work. > single host, the persistent file is redundant to the domain XML (as > long as uuid+parent is in the xml) and in case of cluster we'd have to Right now you don't have any info about the parent device in the domain XML and such data would only exist in the XML if we all agreed on auto-creating mdevs, in which case persistent configs in nodedev would be unnecessary and vice-versa. > copy all possible VM mdev definitions to all the hosts. ^For mdev configs, you might be better off with creating them explicitly than copying configs, simply because given the information the XML has, you might conflict with UUIDs between hosts, so you'd have to take care for that. Parents have different PCI addresses that most probably wouldn't match across hosts, so from automation point of view, I think writing a stub recreating the whole set of devices/configs might actually be easier than copying & handling them (solely because the 2 things left - after the ones I mentioned - in the XML are the vgpu type and IOMMU group number which AFAIK cannot be requested explicitly). > > The idea works nicely if you had such definitions accessible in the > cluster and could define a group of devices (gpu+soundcard, single > mdev, single vf, ...) that would later be assigned to a VM (let's hope > kubevirt can get there). > > As for automatic creation, I think it's on the "nice to have" level. > So far libvirt is close to useless when working with mdevs as all the > data is in the same sysfs place where create/delete endpoints are - as > mentioned earlier, we can just get the data and do everything directly > from there instead of dealing with XML and bunch of new API calls. > Having at least some *configurable* auto create policy might add some ^this is the thing we constantly keep discussing as everyone has a slightly different angle of view - libvirt does not implement any kind of policy, therefore the only "configuration" would be the PCI parent placement - you say what to do and we do it, no logic in it, that's it. Now, I don't understand taking care of the guesswork for the user in the simplest manner possible as policy rather as a mere convenience, be it just for developers and testers, but even that might apparently be perceived as a policy and therefore unacceptable. I still stand by idea of having auto-creation as unfortunately, I sort of still fail to understand what the negative implications of having it are - is that it would get just unnecessarily too complex to maintain in the future that we would regret it or that we'd get a huge amount of follow-up requests for extending the feature or is it just that simply the interpretation of auto-create == policy?

Thanks, Erik

> > ^this is the thing we constantly keep discussing as everyone has a slightly > different angle of view - libvirt does not implement any kind of policy, > therefore the only "configuration" would be the PCI parent placement - you say > what to do and we do it, no logic in it, that's it. Now, I don't understand > taking care of the guesswork for the user in the simplest manner possible as > policy rather as a mere convenience, be it just for developers and testers, but > even that might apparently be perceived as a policy and therefore unacceptable. > > I still stand by idea of having auto-creation as unfortunately, I sort of still > fail to understand what the negative implications of having it are - is that it > would get just unnecessarily too complex to maintain in the future that we would > regret it or that we'd get a huge amount of follow-up requests for extending the > feature or is it just that simply the interpretation of auto-create == policy? The increasing complexity of the qemu driver is a significant concern with adding policy based logic to the code. THinking about this though, if we provide the inactive node device feature, then we can avoid essentially all new code and complexity QEMU driver, and still support auto-create. ie, in the domain XML we just continue to have the exact same XML that we already have today for mdevs, but with a single new attribute autocreate=yes|no <devices> <hostdev mode='subsystem' type='mdev' model='vfio-pci' autocreate="yes"> <source> <address uuid='c2177883-f1bb-47f0-914d-32a22e3a8804'>

</source> </hostdev> </devices> In the QEMU driver, then the only change required is if (def->autocreate) virNodeDeviceCreate(dev)

On Thu, 22 Jun 2017 17:14:48 +0200 Erik Skultety <eskultet(a)redhat.com&gt; wrote: > [...] > > > > > > ^this is the thing we constantly keep discussing as everyone has a slightly > > > different angle of view - libvirt does not implement any kind of policy, > > > therefore the only "configuration" would be the PCI parent placement - you say > > > what to do and we do it, no logic in it, that's it. Now, I don't understand > > > taking care of the guesswork for the user in the simplest manner possible as > > > policy rather as a mere convenience, be it just for developers and testers, but > > > even that might apparently be perceived as a policy and therefore unacceptable. > > > > > > I still stand by idea of having auto-creation as unfortunately, I sort of still > > > fail to understand what the negative implications of having it are - is that it > > > would get just unnecessarily too complex to maintain in the future that we would > > > regret it or that we'd get a huge amount of follow-up requests for extending the > > > feature or is it just that simply the interpretation of auto-create == policy? > > > > The increasing complexity of the qemu driver is a significant concern with > > adding policy based logic to the code. THinking about this though, if we > > provide the inactive node device feature, then we can avoid essentially > > all new code and complexity QEMU driver, and still support auto-create. > > > > ie, in the domain XML we just continue to have the exact same XML that > > we already have today for mdevs, but with a single new attribute > > autocreate=yes|no > > > > <devices> > > <hostdev mode='subsystem' type='mdev' model='vfio-pci' autocreate="yes"> > > <source> > > <address uuid='c2177883-f1bb-47f0-914d-32a22e3a8804'> > > So, just for clarification of the concept, the device with ^this UUID will have > had to be defined by the nodedev API by the time we start to edit the domain > XML in this manner in which case the only thing the autocreate=yes would do is > to actually create the mdev according to the nodedev config, right? Continuing > with that thought, if UUID doesn't refer to any of the inactive configs it will > be an error I suppose? What about the fact that only one vgpu type can live on > the GPU? even if you can successfully identify a device using the UUID in this > way, you'll still face the problem, that other types might be currently > occupying the GPU and need to be torn down first, will this be automated as > well in what you suggest? I assume not. > > > </source> > > </hostdev> > > </devices> > > > > In the QEMU driver, then the only change required is > > > > if (def->autocreate) > > virNodeDeviceCreate(dev) > > Aha, so if a device gets torn down on shutdown, we won't face the problem with > some other devices being active, all of them will have to be in the inactive > state because they got torn down during the last shutdown - that would work. I'm not familiar with how inactive devices would be defined in the nodedev API, would someone mind explaining or providing an example please? I don't understand where the metadata is stored that describes the what and where of a given UUID. Thanks,

Alex -- libvir-list mailing list libvir-list(a)redhat.com https://www.redhat.com/mailman/listinfo/libvir-list

On Thu, 22 Jun 2017 17:14:48 +0200 Erik Skultety <eskultet(a)redhat.com&gt; wrote: > [...] >>> >>> ^this is the thing we constantly keep discussing as everyone has a slightly >>> different angle of view - libvirt does not implement any kind of policy, >>> therefore the only "configuration" would be the PCI parent placement - you say >>> what to do and we do it, no logic in it, that's it. Now, I don't understand >>> taking care of the guesswork for the user in the simplest manner possible as >>> policy rather as a mere convenience, be it just for developers and testers, but >>> even that might apparently be perceived as a policy and therefore unacceptable. >>> >>> I still stand by idea of having auto-creation as unfortunately, I sort of still >>> fail to understand what the negative implications of having it are - is that it >>> would get just unnecessarily too complex to maintain in the future that we would >>> regret it or that we'd get a huge amount of follow-up requests for extending the >>> feature or is it just that simply the interpretation of auto-create == policy? >> >> The increasing complexity of the qemu driver is a significant concern with >> adding policy based logic to the code. THinking about this though, if we >> provide the inactive node device feature, then we can avoid essentially >> all new code and complexity QEMU driver, and still support auto-create. >> >> ie, in the domain XML we just continue to have the exact same XML that >> we already have today for mdevs, but with a single new attribute >> autocreate=yes|no >> >> <devices> >> <hostdev mode='subsystem' type='mdev' model='vfio-pci' autocreate="yes"> >> <source> >> <address uuid='c2177883-f1bb-47f0-914d-32a22e3a8804'> > > So, just for clarification of the concept, the device with ^this UUID will have > had to be defined by the nodedev API by the time we start to edit the domain > XML in this manner in which case the only thing the autocreate=yes would do is > to actually create the mdev according to the nodedev config, right? Continuing > with that thought, if UUID doesn't refer to any of the inactive configs it will > be an error I suppose? What about the fact that only one vgpu type can live on > the GPU? even if you can successfully identify a device using the UUID in this > way, you'll still face the problem, that other types might be currently > occupying the GPU and need to be torn down first, will this be automated as > well in what you suggest? I assume not. > >> </source> >> </hostdev> >> </devices> >> >> In the QEMU driver, then the only change required is >> >> if (def->autocreate) >> virNodeDeviceCreate(dev) > > Aha, so if a device gets torn down on shutdown, we won't face the problem with > some other devices being active, all of them will have to be in the inactive > state because they got torn down during the last shutdown - that would work. I'm not familiar with how inactive devices would be defined in the nodedev API, would someone mind explaining or providing an example please? I don't understand where the metadata is stored that describes the what and where of a given UUID. Thanks,

[...] > > > > ^this is the thing we constantly keep discussing as everyone has a slightly > > different angle of view - libvirt does not implement any kind of policy, > > therefore the only "configuration" would be the PCI parent placement - you say > > what to do and we do it, no logic in it, that's it. Now, I don't understand > > taking care of the guesswork for the user in the simplest manner possible as > > policy rather as a mere convenience, be it just for developers and testers, but > > even that might apparently be perceived as a policy and therefore unacceptable. > > > > I still stand by idea of having auto-creation as unfortunately, I sort of still > > fail to understand what the negative implications of having it are - is that it > > would get just unnecessarily too complex to maintain in the future that we would > > regret it or that we'd get a huge amount of follow-up requests for extending the > > feature or is it just that simply the interpretation of auto-create == policy? > > The increasing complexity of the qemu driver is a significant concern with > adding policy based logic to the code. THinking about this though, if we > provide the inactive node device feature, then we can avoid essentially > all new code and complexity QEMU driver, and still support auto-create. > > ie, in the domain XML we just continue to have the exact same XML that > we already have today for mdevs, but with a single new attribute > autocreate=yes|no > > <devices> > <hostdev mode='subsystem' type='mdev' model='vfio-pci' autocreate="yes"> > <source> > <address uuid='c2177883-f1bb-47f0-914d-32a22e3a8804'> So, just for clarification of the concept, the device with ^this UUID will have had to be defined by the nodedev API by the time we start to edit the domain XML in this manner in which case the only thing the autocreate=yes would do is to actually create the mdev according to the nodedev config, right? Continuing with that thought, if UUID doesn't refer to any of the inactive configs it will be an error I suppose? What about the fact that only one vgpu type can live on the GPU? even if you can successfully identify a device using the UUID in this way, you'll still face the problem, that other types might be currently occupying the GPU and need to be torn down first, will this be automated as well in what you suggest? I assume not.

> </source> > </hostdev> > </devices> > > In the QEMU driver, then the only change required is > > if (def->autocreate) > virNodeDeviceCreate(dev) Aha, so if a device gets torn down on shutdown, we won't face the problem with some other devices being active, all of them will have to be in the inactive state because they got torn down during the last shutdown - that would work.

>> So, just for clarification of the concept, the device with ^this UUID will have >> had to be defined by the nodedev API by the time we start to edit the domain >> XML in this manner in which case the only thing the autocreate=yes would do is >> to actually create the mdev according to the nodedev config, right? Continuing >> with that thought, if UUID doesn't refer to any of the inactive configs it will >> be an error I suppose? What about the fact that only one vgpu type can live on >> the GPU? even if you can successfully identify a device using the UUID in this >> way, you'll still face the problem, that other types might be currently >> occupying the GPU and need to be torn down first, will this be automated as >> well in what you suggest? I assume not. > > Technically we shouldn't need the node device to exist at the time we > define the XML - only at the time we start the guest, does the node > device have to exist. eg same way you list a virtual network as the > source of a guest NIC, but that virtual network doesn't have to actually > have been defined & started until the guest starts. > > If there are constraints that a pGPU can only support a certain combination > of vGPUs at any single point in time, doesn't the kernel already enforce > that when you try to create the vGPU in sysfs. IOW, we merely need to try > to create the vGPU, and if the kernel mdev driver doesn't allow you to mix > that with the other vGPUs that already exist, then we'd just report an > error from virNodeDeviceCreate, and that'd get propagated back as the > error for the virDomainCreate call. > >> >>> </source> >>> </hostdev> >>> </devices> >>> >>> In the QEMU driver, then the only change required is >>> >>> if (def->autocreate) >>> virNodeDeviceCreate(dev) >> >> Aha, so if a device gets torn down on shutdown, we won't face the problem with >> some other devices being active, all of them will have to be in the inactive >> state because they got torn down during the last shutdown - that would work. > > I'm not sure what the relationship with other active devices is relevant > here. The virNodeDevicePtr we're accesing here is a single vGPU - if other > running guests have further vGPUs on the same pGPU, that's not really > relevant. Each vGPU is created/deleted as required. I think he's talking about devices that were previously used by other domains that are no longer active. Since they're also automatically destroyed, they're not a problem.

On Fri, Jun 16, 2017 at 11:02:55AM -0600, Alex Williamson wrote: > On Fri, 16 Jun 2017 11:32:04 -0400 > Laine Stump <laine(a)redhat.com&gt; wrote: > > > On 06/15/2017 02:42 PM, Alex Williamson wrote: > > > On Thu, 15 Jun 2017 09:33:01 +0100 > > > "Daniel P. Berrange" <berrange(a)redhat.com&gt; wrote: > > > > > >> On Thu, Jun 15, 2017 at 12:06:43AM +0200, Erik Skultety wrote: > > >>> Hi all, > > >>> > > >>> so there's been an off-list discussion about finally implementing creation of > > >>> mediated devices with libvirt and it's more than desired to get as many opinions > > >>> on that as possible, so please do share your ideas. This did come up already as > > >>> part of some older threads ([1] for example), so this will be a respin of the > > >>> discussions. Long story short, we decided to put device creation off and focus > > >>> on the introduction of the framework as such first and build upon that later, > > >>> i.e. now. > > >>> > > >>> [1] https://www.redhat.com/archives/libvir-list/2017-February/msg00177.html > > >>> > > >>> ======================================== > > >>> PART 1: NODEDEV-DRIVER > > >>> ======================================== > > >>> > > >>> API-wise, device creation through the nodedev driver should be pretty > > >>> straightforward and without any issues, since virNodeDevCreateXML takes an XML > > >>> and does support flags. Looking at the current device XML: > > >>> > > >>> <device> > > >>> <name>mdev_0cce8709_0640_46ef_bd14_962c7f73cc6f</name> > > >>> <path>/sys/devices/pci0000:00/.../0cce8709-0640-46ef-bd14-962c7f73cc6f</path> > > >>> <parent>pci_0000_03_00_0</parent> > > >>> <driver> > > >>> <name>vfio_mdev</name> > > >>> </driver> > > >>> <capability type='mdev'> > > >>> <type id='nvidia-11'/> > > >>> <iommuGroup number='13'/> > > >>> <uuid>UUID<uuid> <!-- optional enhancement, see below --> > > >>> </capability> > > >>> </device> > > >>> > > >>> We can ignore <path>,<driver>,<iommugroup> elements, since these are useless > > >>> during creation. We also cannot use <name> since we don't support arbitrary > > >>> names and we also can't rely on users providing a name in correct form which we > > >>> would need to further parse in order to get the UUID. > > >>> So since the only thing missing to successfully use create an mdev using XML is > > >>> the UUID (if user doesn't want it to be generated automatically), how about > > >>> having a <uuid> subelement under <capability> just like PCIs have <domain> and > > >>> friends, USBs have <bus> & <device>, interfaces have <address> to uniquely > > >>> identify the device even if the name itself is unique. > > >>> Removal of a device should work as well, although we might want to > > >>> consider creating a *Flags version of the API. > > >>> > > >>> ============================================================= > > >>> PART 2: DOMAIN XML & DEVICE AUTO-CREATION, NO POLICY INVOLVED! > > >>> ============================================================= > > >>> > > >>> There were some doubts about auto-creation mentioned in [1], although they > > >>> weren't specified further. So hopefully, we'll get further in the discussion > > >>> this time. > > >>> > > >>> From my perspective there are two main reasons/benefits to that: > > >>> > > >>> 1) Convenience > > >>> For apps like virt-manager, user will want to add a host device transparently, > > >>> "hey libvirt, I want an mdev assigned to my VM, can you do that". Even for > > >>> higher management apps, like oVirt, even they might not care about the parent > > >>> device at all times and considering that they would need to enumerate the > > >>> parents, pick one, create the device XML and pass it to the nodedev driver, IMHO > > >>> it would actually be easier and faster to just do it directly through sysfs, > > >>> bypassing libvirt once again.... > > >> > > >> The convenience only works if the policy we've provided in libvirt actually > > >> matches the policy the application wants. I think it is quite likely that with > > >> cloud the mdevs will be created out of band from the domain startup process. > > >> It is possible the app will just have a fixed set of mdevs pre-created when > > >> the host starts up. Or that the mgmt app wants the domain startup process to > > >> be a two phase setup, where it first allocates the resources needed, and later > > >> then tries to start the guest. This is why I keep saying that putting this kind > > >> of "convenient" policy in libvirt is a bad idea - it is essentially just putting > > >> a bit of virt-manager code into libvirt - more advanced apps will need more > > >> flexibility in this area. > > >> > > >>> 2) Future domain migration > > >>> Suppose now that the mdev backing physical devices support state dump and > > >>> reload. Chances are, that the corresponding mdev doesn't even exist or has a > > >>> different UUID on the destination, so libvirt would do its best to handle this > > >>> before the domain could be resumed. > > >> > > >> This is not an unusual scenario - there are already many other parts of the > > >> device backend config that need to change prior to migration, especially for > > >> anything related to host devices, so apps already have support for doing > > >> this, which is more flexible & convenient becasue it doesn't tie creation of > > >> the mdevs to running of the migrate command. > > >> > > >> IOW, I'm still against adding any kind of automatic creation policy for > > >> mdevs in libvirt. Just provide the node device API support. > > > > > > I'm not super clear on the extent of what you're against here, is it > > > all forms of device creation or only a placement policy? Are you > > > against any form of having the XML specify the non-instantiated mdev > > > that it wants? We've clearly made an important step with libvirt > > > supporting pre-created mdevs, but as a user of that support I find it > > > incredibly tedious. I typically do a dumpxml, copy out the UUID, > > > wonder what type of device it might have been last time, create it, > > > start the domain and cross my fingers. Pre-creating mdev devices is not > > > really practical, I might have use cases where I want multiple low-end > > > mdev devices and another where I have a single high-end device. Those > > > cannot exist at the same time. Requiring extensive higher level > > > management tools is not really an option either, I'm not going to > > > install oVirt on my desktop/laptop just so I can launch a GVT-g VM once > > > in a while (no offense). So I really hope that libvirt itself can > > > provide some degree of mdev creation. > > > > > > Maybe there can be something in between the "all child devices must be > > pre-created" and "a child device will be automatically created on an > > automatically chosen parent device as needed". In particular, we could > > forego the "automatically chosen parent device" part of that. The guest > > configuration could simply contain the PCI address of the parent and the > > desired type of the child. If we did this there wouldn't be any policy > > decision to make - all the variables are determined - but it would make > > life easier for people running small hosts (i.e. no oVirt/Openstack, a > > single mdev parent device). Openstack and oVirt (and whoever) would of > > course be free to ignore this and pre-create pools of devices themselves > > in the name of more precise control and better predictability (just as, > > for example, OpenStack ignores libvirt's "pools of hostdev network > > devices" and instead manages the pool of devices itself and uses > > <interface type='hostdev'> directly). > > This seems not that substantially different from managed='yes' on a > vfio hostdev to me. It makes the device available to the VM before it > starts and returns it after. In one case that's switching the binding > on an existing device, in another it's creating and removing. Once > again, I can't tell from Dan's response if he's opposed to this entire > idea or just the aspects where libvirt needs to impose a policy > decision. For me personally, the functionality difference is quite > substantial. I'm fine with libvirt having APIs in the node device APIs to enable create/delete with libvirt, as well as using managed=yes in the same manner that we do for regular PCI devices (the bind/unbind to vfio or pci-back) I'm only against the creation/deletion of mdevs, as a side effect of starting/stopping the guest.

On 06/14/2017 06:06 PM, Erik Skultety wrote: > Hi all, > > so there's been an off-list discussion about finally implementing creation of > mediated devices with libvirt and it's more than desired to get as many opinions > on that as possible, so please do share your ideas. This did come up already as > part of some older threads ([1] for example), so this will be a respin of the > discussions. Long story short, we decided to put device creation off and focus > on the introduction of the framework as such first and build upon that later, > i.e. now. > > [1] https://www.redhat.com/archives/libvir-list/2017-February/msg00177.html > > ======================================== > PART 1: NODEDEV-DRIVER > ======================================== > > API-wise, device creation through the nodedev driver should be pretty > straightforward and without any issues, since virNodeDevCreateXML takes an XML > and does support flags. Looking at the current device XML: > > <device> > <name>mdev_0cce8709_0640_46ef_bd14_962c7f73cc6f</name> > <path>/sys/devices/pci0000:00/.../0cce8709-0640-46ef-bd14-962c7f73cc6f</path> > <parent>pci_0000_03_00_0</parent> > <driver> > <name>vfio_mdev</name> > </driver> > <capability type='mdev'> > <type id='nvidia-11'/> > <iommuGroup number='13'/> > <uuid>UUID<uuid> <!-- optional enhancement, see below --> > </capability> > </device> > > We can ignore <path>,<driver>,<iommugroup> elements, since these are useless > during creation. We also cannot use <name> since we don't support arbitrary > names and we also can't rely on users providing a name in correct form which we > would need to further parse in order to get the UUID. > So since the only thing missing to successfully use create an mdev using XML is > the UUID (if user doesn't want it to be generated automatically), how about > having a <uuid> subelement under <capability> just like PCIs have <domain> and > friends, USBs have <bus> & <device>, interfaces have <address> to uniquely > identify the device even if the name itself is unique. > Removal of a device should work as well, although we might want to > consider creating a *Flags version of the API. Has any thought been put towards creating an mdev pool modeled after the Storage Pool? Similar to how vHBA's are created from a Storage Pool XML definition. That way XML could be defined to keep track of a lot of different things that you may need and would require only starting the pool in order to access. Placed "appropriately" - the mdev's could already be available by the time node device state initialization occurs too since the pool would conceivably been created/defined using data from the physical device and the calls to create the virtual devices would have occurred. Much easier to add logic to a new driver/pool mgmt to handle whatever considerations there are than adding logic into the existing node device driver.