Disabling PCI "hot-unplug" for a guest (and/or a single PCI device)
Although I've never experienced it, due to not running Windows guests, I've recently learned that a Windows guest permits a user (hopefully only one with local admin privileges??!) to "hot-unplug" any PCI device. I've also learned that some hypervisor admins don't want to permit admins of the virtual machines they're managing to unplug PCI devices. I believe this is impossible to prevent on an i440fx-based machinetype, and can only be done on a q35-based machinetype by assigning the devices to the root bus (so that they are seen as integrated devices) rather than to a pcie-root-port. But when libvirt is assigning PCI addresses to devices in a q35-base guest, it will *always* assign a PCIe device to a pcie-root-port specifically so that hotplug is possible (this was done to maintain functional parity with i440fx guests, where all PCI slots support hotplug). To make the above-mentioned admins happy, we need to make it possible to (easily) create guest configurations for q35-based virtual machines where the PCI devices can't be hot-unplugged by the guest OS. Thinking in the context of a management platform (e.g. OpenStack or ovirt) that goes through libvirt to use QEMU (and forgetting about i440fx, concentrating only on q35), I can think of a few different ways this could be done: 1) Rather than leaving the task of assignung the PCI addresses of devices to libvirt (which is what essentially *all* management apps that use libvirt currently do), the management application could itself directly assign the PCI addressed of all devices to be slots on pcie.0. This is problematic because once a management application has taken over the PCI address assignment of a single device, it must learn the rules of what type of device can be plugged into what type of PCI controller (including plugging in new controllers when necessary), and keep track of which slots on which PCI controllers are already in use - effectively tossing that part of libvirt's functionality / embedded knowledge / usefulness to management applications out the window. It's even more of a problem for management applications that have no provision for manually assigning PCI addresses - virt-manager for example only supports this by using "XML mode" where the froopy point-click UI is swapped out for an edit window where the user is simply presented with the full XML for a device and allowed to tweak it around as they see fit (including duplicate addresses, plugging the wrong kind of device into the wrong slot, referencing non-existent controllers, etc). (NB: you could argue that management could just take over PCI address assignment in the case of wanting hotplug disabled, and only care about / support pcie.0 (which makes the task much easier, since you just ignore the existence of any other PCI controllers, leaving you with a homogenous array of 32 slot x 8 functions, but becomes much more complicated if you want to allow a mix of hotpluggable and non-hotpluggable devices, and you *know* someone will) 2) libvirt could gain a knob "somewhere" in the domain XML to force a single device, or all devices, to be assigned to a PCI address on pcie.0 rather than on a pcie-root-port. This could be thought of as a "hint" about device placement, as well as extra validation in the case that a PCI address has been manually assigned. So, for example, let's say a "hotplug='disable'" option is added somewhere at the top level of the domain (maybe "<hotplug enable='no'/>" inside <features> or something like that); when PCI addresses are assigned by libvirt, it would attempt to find a slot on a controller that didn't support hotplug. And/or a similar knob could be added to each device. In both cases, the setting would be used both when assigning PCI addresses and also to validate user-provided PCI addresses to assure that the desired criterion was met (otherwise someone would manually select a PCI address on a controller that supported hotplug, but then set "hotplug='disabled'" and expect hotplug to be magically disabled on the slot). Some of you will remember that I proposed such a knob for libvirt a few years ago when we were first fleshing out support for QEMU's PCI Express controllers and the Q35 machinetype, and it was rejected as "libvirt dictating policy". Of course at that time there weren't actual users demanding the functionality, and now there are. Aside from that, all I can say is that it isn't libvirt dictating this policy, it's the user of libvirt, and libvirt is just following directions :-) (and that I really really dislike the idea of a forced handover of the entire task of assigning/managing device PCI addresses to management apps just because they decide they want to disable guest-initiated hotplug 3) qemu could add a "hotpluggable=no" commandline option to all PCI devices (including vfio-pci) and then do whatever is necessary to make sure this is honored in the emulated hardware (is it possible to set this on a per-slot basis in a PCI controller? Or must it be done for an entire controller? I suppose it's not as much of an issue for pcie-root-port, as long as you're not using multiple functions). libvirt would then need to add this option to the XML for each device, and management applications would need to set it - it would essentially look the same to the management application, but it would be implemented differently - instead of libvirt using that flag to make a choice about which slot to assign, it would assign PCI addresses in the same manner as before, and use the libvirt XML flag to set a QEMU commandline flag for the device. The upside of this is that we would be disabling hotplug by "disabling hotplug" rather than by "assigning the device to a slot that coincidentally doesn't support hotplug", making it all more orthogonal - everything else in a guest's config could remain exactly the same while enabling/disabling hotplug. (Another upside is that it could possibly be made to work for i440fx machine types, but we're not supposed to care about that any more, so I won't mention it :-)) The downside is that it requires a new feature in QEMU (whose difficulty/feasibility I have 0 knowledge of), so there are 3 layers of work rather than 2. So does anyone have any different (and hopefully better) idea of how to do this? Arguments for/against the 3 possibilities I've listed here?
On Mon, Feb 03, 2020 at 05:19:51PM -0500, Laine Stump wrote:
3) qemu could add a "hotpluggable=no" commandline option to all PCI devices (including vfio-pci) and then do whatever is necessary to make sure this is honored in the emulated hardware (is it possible to set this on a per-slot basis in a PCI controller? Or must it be done for an entire controller?
I think it's possible on a per-slot basis, yes.
On Tue, Feb 4, 2020 at 11:26 AM Michael S. Tsirkin <mst@redhat.com> wrote:
On Mon, Feb 03, 2020 at 05:19:51PM -0500, Laine Stump wrote:
3) qemu could add a "hotpluggable=no" commandline option to all PCI devices (including vfio-pci) and then do whatever is necessary to make sure this is honored in the emulated hardware (is it possible to set this on a per-slot basis in a PCI controller? Or must it be done for an entire controller?
I think it's possible on a per-slot basis, yes.
There's a "Hot-Plug Capable" option in Slot Capability register, so we can switch it off. But it's only for pcie devices, can't say anything about conventional pci. Best regards, Julia Suvorova.
On Tue, Feb 04, 2020 at 05:13:54PM +0100, Julia Suvorova wrote:
On Tue, Feb 4, 2020 at 11:26 AM Michael S. Tsirkin <mst@redhat.com> wrote:
On Mon, Feb 03, 2020 at 05:19:51PM -0500, Laine Stump wrote:
3) qemu could add a "hotpluggable=no" commandline option to all PCI devices (including vfio-pci) and then do whatever is necessary to make sure this is honored in the emulated hardware (is it possible to set this on a per-slot basis in a PCI controller? Or must it be done for an entire controller?
I think it's possible on a per-slot basis, yes.
There's a "Hot-Plug Capable" option in Slot Capability register, so we can switch it off. But it's only for pcie devices, can't say anything about conventional pci.
Best regards, Julia Suvorova.
For conventional PCI, we can drop SHPC capability and remove the eject method from ACPI.
On Tue, Feb 04, 2020 at 11:35:37AM -0500, Michael S. Tsirkin wrote:
On Tue, Feb 04, 2020 at 05:13:54PM +0100, Julia Suvorova wrote:
On Tue, Feb 4, 2020 at 11:26 AM Michael S. Tsirkin <mst@redhat.com> wrote:
On Mon, Feb 03, 2020 at 05:19:51PM -0500, Laine Stump wrote:
3) qemu could add a "hotpluggable=no" commandline option to all PCI devices (including vfio-pci) and then do whatever is necessary to make sure this is honored in the emulated hardware (is it possible to set this on a per-slot basis in a PCI controller? Or must it be done for an entire controller?
I think it's possible on a per-slot basis, yes.
There's a "Hot-Plug Capable" option in Slot Capability register, so we can switch it off. But it's only for pcie devices, can't say anything about conventional pci.
Best regards, Julia Suvorova.
For conventional PCI, we can drop SHPC capability and remove the eject method from ACPI.
Before considering this, is there any compelling reason to care about this for PCI ? Currently with i440fx there's no direct representation of the 32 slots as objects in either QEMU or libvirt. So extending this to allow disabling hotplug for i440fx PCI slots is going to need much more config work for QEMU, libvirt and mgmt apps. Personally I'd only do this for PCIe until there's a clear requirement given for legacy PCI support 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 Wed, Feb 05, 2020 at 11:36:37AM +0000, Daniel P. Berrangé wrote:
On Tue, Feb 04, 2020 at 11:35:37AM -0500, Michael S. Tsirkin wrote:
On Tue, Feb 04, 2020 at 05:13:54PM +0100, Julia Suvorova wrote:
On Tue, Feb 4, 2020 at 11:26 AM Michael S. Tsirkin <mst@redhat.com> wrote:
On Mon, Feb 03, 2020 at 05:19:51PM -0500, Laine Stump wrote:
3) qemu could add a "hotpluggable=no" commandline option to all PCI devices (including vfio-pci) and then do whatever is necessary to make sure this is honored in the emulated hardware (is it possible to set this on a per-slot basis in a PCI controller? Or must it be done for an entire controller?
I think it's possible on a per-slot basis, yes.
There's a "Hot-Plug Capable" option in Slot Capability register, so we can switch it off. But it's only for pcie devices, can't say anything about conventional pci.
Best regards, Julia Suvorova.
For conventional PCI, we can drop SHPC capability and remove the eject method from ACPI.
Before considering this, is there any compelling reason to care about this for PCI ?
Not that I know. I simply answered Julia's question.
Currently with i440fx there's no direct representation of the 32 slots as objects in either QEMU or libvirt. So extending this to allow disabling hotplug for i440fx PCI slots is going to need much more config work for QEMU, libvirt and mgmt apps. Personally I'd only do this for PCIe until there's a clear requirement given for legacy PCI support too.
Makes sense to me.
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 Mon, Feb 03, 2020 at 05:19:51PM -0500, Laine Stump wrote:
Although I've never experienced it, due to not running Windows guests, I've recently learned that a Windows guest permits a user (hopefully only one with local admin privileges??!) to "hot-unplug" any PCI device. I've also learned that some hypervisor admins don't want to permit admins of the virtual machines they're managing to unplug PCI devices. I believe this is impossible to prevent on an i440fx-based machinetype, and can only be done on a q35-based machinetype by assigning the devices to the root bus (so that they are seen as integrated devices) rather than to a pcie-root-port. But when libvirt is assigning PCI addresses to devices in a q35-base guest, it will *always* assign a PCIe device to a pcie-root-port specifically so that hotplug is possible (this was done to maintain functional parity with i440fx guests, where all PCI slots support hotplug).
After speaking with Alex & Laine on IRC, I learnt some further relevant points - In a "typical" physical machine PCI slots will not be marked as hotpluggable - /sys/bus/pci/slots only has 2 entries on my HP DL180, corresponding to unused physical PCI slots - QEMU is hardcoded to report all pci & pcie-root-ports as hotpluggable. So /sys/bus/pci/slots on i440fx has 31 entries, one for every device, while on q35 it has one entry for every pcie-root-port IIUC. - It is conceptually possible to enhance pcie-root-port device to allow its hotplug capability to be toggled. Alternatively a parallel pcie-root-port-nohotplug device could be created. The end result would be the same from guest POV - The vfio-pci device has a companion vfio-pci-nohotplug device. The difference is simply whether the QEMU DEviceClass has the "hotpluggable" attribute set, and is separate from whether the PCI(e) root port has hotplug enabled The last point here about vfio-pci si particularly important, as it shows libvirt needs to be capable of tracking hotpluggability independantly on the PCI port and the PCI device attached to the port.
1) Rather than leaving the task of assignung the PCI addresses of devices to libvirt (which is what essentially *all* management apps that use libvirt currently do), the management application could itself directly assign the PCI addressed of all devices to be slots on pcie.0.
[snip] This is essentially a hack to work around the fact the the pcie-root-port is hardcoded to report itself as hotpluggable. As such I don't consider this is serious long term solution. If you absolutely cannot wait for a newer libvirt/QEMU, this solution could be used as a quick hack for mgmt apps, but long term we need todo better.
2) libvirt could gain a knob "somewhere" in the domain XML to force a single device, or all devices, to be assigned to a PCI address on pcie.0 rather than on a pcie-root-port. This could be thought of as a "hint" about device placement, as well as extra validation in the case that a PCI address has been manually assigned. So, for example, let's say a "hotplug='disable'" option is added somewhere at the top level of the domain (maybe "<hotplug enable='no'/>" inside <features> or something like that); when PCI addresses are assigned by libvirt, it would attempt to find a slot on a controller that didn't support hotplug. And/or a similar knob could be added to each device. In both cases, the setting would be used both when assigning PCI addresses and also to validate user-provided PCI addresses to assure that the desired criterion was met (otherwise someone would manually select a PCI address on a controller that supported hotplug, but then set "hotplug='disabled'" and expect hotplug to be magically disabled on the slot).
Essentially this is the using "hotpluggable=yes|no" on the device as a policy knob to control device placement, to again workaround the fact that pcie-root-port are hardcoded to always report themselves as hotpluggable. So I'd think this should be ruled out for the same reason as option 1. It has a second downside though. As we see from vfio-pci-nohotplug, there is a valid use case for a "hotpluggable=yes|no" attribute on a device for controlling a specific hardware config choice in QEMU. We don't want to overload this attribute to both control use of vfio-pci-nohotplug, and also be a policy knob for device placement.
3) qemu could add a "hotpluggable=no" commandline option to all PCI devices (including vfio-pci) and then do whatever is necessary to make sure this is honored in the emulated hardware (is it possible to set this on a per-slot basis in a PCI controller? Or must it be done for an entire controller? I suppose it's not as much of an issue for pcie-root-port, as long as you're not using multiple functions). libvirt would then need to add this option to the XML for each device, and management applications would need to set it - it would essentially look the same to the management application, but it would be implemented differently - instead of libvirt using that flag to make a choice about which slot to assign, it would assign PCI addresses in the same manner as before, and use the libvirt XML flag to set a QEMU commandline flag for the device.
I think this, or something close to it is the desirable way forward here, as it gives us more explicit control over what the emulated hardware actually advertizes. So instead of trying to workaround limitations of QEMU, we'd be working /with/ QEMU to improve its feature offerings. In QEMU pcie-root-port either needs to gain a hotpluggable=yes|no attribute, or a second pcie-root-port-nohotplug needs adding. Withever of these two approaches are taken in QEMU, this can be controlled from libvirt via an attribute on the controller.eg <controller type='pci' model='pcie-root-port' hotpluggable="no|yes"/> This hotpluggable attribute can be mapped to whichever CLI syntax QEMU wants to support. This alone is probably sufficient for the Windows problem motivating this thread. There already exists the vfio-pci-nohotplug device, but this is not exposed by libvirt. So we can add an attribute to <hostdev> to control its use. The remaining question is whether there's any compelling reason to add non-hotpluggable variants of other devices, virtio-net-pci-nohotplug ? I'd probably /not/ do this, unless there's a clear compelling benefit it gives which can't be achieved already via the pcie-root-port hotpluggability controls. For management applications, with Q35 we already recommend that they explicitly add *many* <controller type='pci' model='pcie-root-port'/> to new guests. Enough to cover all the initial cold-plugged devices, and enough spare ports to enable future hotplug of extra devices. OpenStack for example will add 32 pcie-root-ports, so that Q35 has approximately the same hotplug capacity as i440fx would have offered. To control hotplug, management apps simply need tweak what they're doing with pcie-root-ports with an extra attribute eg, consider there were 4 devices on the initially booted VM which need hotplug disabled, and we still want freedom to hotplug 2 extra devices at runtime. <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="yes"/> <controller type='pci' model='pcie-root-port' hotplug="yes"/> This is quite easy, as applications still do *not* have to taken on responsibility for full PCI device addressing. They merely need to be able to count how many PCI devices they're using. The only "gotcha" is if they forget about the auto-added USB, VGA and Balloon devices, but that's not a big deal. 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 2/4/20 1:43 PM, Daniel P. Berrangé wrote:
On Mon, Feb 03, 2020 at 05:19:51PM -0500, Laine Stump wrote:
Although I've never experienced it, due to not running Windows guests, I've recently learned that a Windows guest permits a user (hopefully only one with local admin privileges??!) to "hot-unplug" any PCI device. I've also learned that some hypervisor admins don't want to permit admins of the virtual machines they're managing to unplug PCI devices. I believe this is impossible to prevent on an i440fx-based machinetype, and can only be done on a q35-based machinetype by assigning the devices to the root bus (so that they are seen as integrated devices) rather than to a pcie-root-port. But when libvirt is assigning PCI addresses to devices in a q35-base guest, it will *always* assign a PCIe device to a pcie-root-port specifically so that hotplug is possible (this was done to maintain functional parity with i440fx guests, where all PCI slots support hotplug).
After speaking with Alex & Laine on IRC, I learnt some further relevant points
- In a "typical" physical machine PCI slots will not be marked as hotpluggable - /sys/bus/pci/slots only has 2 entries on my HP DL180, corresponding to unused physical PCI slots
- QEMU is hardcoded to report all pci & pcie-root-ports as hotpluggable. So /sys/bus/pci/slots on i440fx has 31 entries, one for every device, while on q35 it has one entry for every pcie-root-port IIUC.
- It is conceptually possible to enhance pcie-root-port device to allow its hotplug capability to be toggled. Alternatively a parallel pcie-root-port-nohotplug device could be created. The end result would be the same from guest POV
- The vfio-pci device has a companion vfio-pci-nohotplug device. The difference is simply whether the QEMU DEviceClass has the "hotpluggable" attribute set, and is separate from whether the PCI(e) root port has hotplug enabled
The last point here about vfio-pci si particularly important, as it shows libvirt needs to be capable of tracking hotpluggability independantly on the PCI port and the PCI device attached to the port.
1) Rather than leaving the task of assignung the PCI addresses of devices to libvirt (which is what essentially *all* management apps that use libvirt currently do), the management application could itself directly assign the PCI addressed of all devices to be slots on pcie.0.
[snip]
This is essentially a hack to work around the fact the the pcie-root-port is hardcoded to report itself as hotpluggable.
As such I don't consider this is serious long term solution. If you absolutely cannot wait for a newer libvirt/QEMU, this solution could be used as a quick hack for mgmt apps, but long term we need todo better.
2) libvirt could gain a knob "somewhere" in the domain XML to force a single device, or all devices, to be assigned to a PCI address on pcie.0 rather than on a pcie-root-port. This could be thought of as a "hint" about device placement, as well as extra validation in the case that a PCI address has been manually assigned. So, for example, let's say a "hotplug='disable'" option is added somewhere at the top level of the domain (maybe "<hotplug enable='no'/>" inside <features> or something like that); when PCI addresses are assigned by libvirt, it would attempt to find a slot on a controller that didn't support hotplug. And/or a similar knob could be added to each device. In both cases, the setting would be used both when assigning PCI addresses and also to validate user-provided PCI addresses to assure that the desired criterion was met (otherwise someone would manually select a PCI address on a controller that supported hotplug, but then set "hotplug='disabled'" and expect hotplug to be magically disabled on the slot).
Essentially this is the using "hotpluggable=yes|no" on the device as a policy knob to control device placement, to again workaround the fact that pcie-root-port are hardcoded to always report themselves as hotpluggable.
So I'd think this should be ruled out for the same reason as option 1.
It has a second downside though. As we see from vfio-pci-nohotplug, there is a valid use case for a "hotpluggable=yes|no" attribute on a device for controlling a specific hardware config choice in QEMU.
We don't want to overload this attribute to both control use of vfio-pci-nohotplug, and also be a policy knob for device placement.
3) qemu could add a "hotpluggable=no" commandline option to all PCI devices (including vfio-pci) and then do whatever is necessary to make sure this is honored in the emulated hardware (is it possible to set this on a per-slot basis in a PCI controller? Or must it be done for an entire controller? I suppose it's not as much of an issue for pcie-root-port, as long as you're not using multiple functions). libvirt would then need to add this option to the XML for each device, and management applications would need to set it - it would essentially look the same to the management application, but it would be implemented differently - instead of libvirt using that flag to make a choice about which slot to assign, it would assign PCI addresses in the same manner as before, and use the libvirt XML flag to set a QEMU commandline flag for the device.
I think this, or something close to it is the desirable way forward here, as it gives us more explicit control over what the emulated hardware actually advertizes. So instead of trying to workaround limitations of QEMU, we'd be working /with/ QEMU to improve its feature offerings.
In QEMU pcie-root-port either needs to gain a hotpluggable=yes|no attribute, or a second pcie-root-port-nohotplug needs adding.
Withever of these two approaches are taken in QEMU, this can be controlled from libvirt via an attribute on the controller.eg
<controller type='pci' model='pcie-root-port' hotpluggable="no|yes"/>
This hotpluggable attribute can be mapped to whichever CLI syntax QEMU wants to support.
This alone is probably sufficient for the Windows problem motivating this thread.
There already exists the vfio-pci-nohotplug device, but this is not exposed by libvirt. So we can add an attribute to <hostdev> to control its use.
The remaining question is whether there's any compelling reason to add non-hotpluggable variants of other devices, virtio-net-pci-nohotplug ?
I'd probably /not/ do this, unless there's a clear compelling benefit it gives which can't be achieved already via the pcie-root-port hotpluggability controls.
For management applications, with Q35 we already recommend that they explicitly add *many*
<controller type='pci' model='pcie-root-port'/>
to new guests. Enough to cover all the initial cold-plugged devices, and enough spare ports to enable future hotplug of extra devices. OpenStack for example will add 32 pcie-root-ports, so that Q35 has approximately the same hotplug capacity as i440fx would have offered.
To control hotplug, management apps simply need tweak what they're doing with pcie-root-ports with an extra attribute
eg, consider there were 4 devices on the initially booted VM which need hotplug disabled, and we still want freedom to hotplug 2 extra devices at runtime.
<controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="no"/> <controller type='pci' model='pcie-root-port' hotplug="yes"/> <controller type='pci' model='pcie-root-port' hotplug="yes"/>
This is quite easy,
Not quite so easy as you might think :-). The problem is that, in our fervor to make Q35 guests "as similar as possible" to 440fx guests, we have made the PCI address assignment code search out a hotplug-capable slot for each unassigned device, and if no hotpluggable slot is available it automatically adds a new root port so that there is a hotplug-capable slot. With the current set of VIR_PCI_CONNECT_TYPE flags, unless we lie to that code and tell it that these new root ports support hotplug, the the devices will be assigned first to the two available hotpluggable root ports (in your example above), and then when those are both used, it will start adding new root ports - the ports with hotplug='no' will never be used. On the other hand, if we change the code to *not* require a hotplug-capable port, then the new devices will just be assigned to slots on the root bus. So the address assignment code is going to need to be re-jiggered. I guess we would need to create yet another PCI controller capability for use by the address assignment code - "Not Hot-Pluggable-But-Still-Okay-For-Auto-Assign-When-The-Guest-Is-Powered-Off". So, a mental exercise - let's say we make a new virDomainPCIConnectFlag called VIR_PCI_CONNECT_AUTO_ASSIGNABLE (that's a bit easier to say than what I had in the previous paragraph). That connect type would be added to all the controllers that currently have CONNECT_HOTPLUGGABLE set: pci-root pci-bridge pcie-root pcie-pci-bridge pcie-root-port pcie-downstream-port We then add a new controller model VIR_DOMAIN_CONTROLLER_PCIE_ROOT_PORT_NO_HOTPLUG and set it as AUTO_ASSIGNABLE, but *not* HOTPLUGGABLE. Now when a device is added to "cold" config, instead of requiring HOTPLUGGABLE, we require AUTO_ASSIGNABLE, which will get the same assignment as before the change (except that the NO_HOTPLUG root ports will also be used). If we actually hotplug a device, then we continue to require the HOTPLUGGABLE connect flag. So that much works, but we still have the problem that in order to add a device to the "cold" config that can later be hotplugged, we have to make sure all of the hotplug='no' controllers are in use. I guess that will work, but it's getting pretty obtuse and complicated just to avoid adding a single flag that says "this device shouldn't be put in a hotplug-capable slot" directly to the XML for the device that you don't want hotplugged. It would be *much* simpler if qemu could provide an attribute for endpoint devices that would disable hotplug for that device, rather than requiring the option to be set on the controller that the device is plugged into. Then libvirt could just have that attribute added to the XML, and the management app would just simply add "hotplug='no'" to each device. It's too bad that the method vfio-pci-nohotplug uses apparently doesn't work for PCIe...
as applications still do *not* have to taken on responsibility for full PCI device addressing. They merely need to be able to count how many PCI devices they're using. The only "gotcha" is if they forget about the auto-added USB, VGA and Balloon devices, but that's not a big deal.
Regards, Daniel
participants (4)
-
Daniel P. Berrangé -
Julia Suvorova -
Laine Stump -
Michael S. Tsirkin