[libvirt] CPU model versioning separate from machine type versioning ?
This post is to raise question about helping use of named CPU models with KVM ie any case not using -cpu host. In the old days (ie before 2018), the world was innocent and we had a nice set of named CPU models that corresponded to different Intel/AMD physical CPU families/generations (lets temporarily ignore the -noTSX fiasco). An application could query libvirt to determine what the host CPU model was/is and use that model name in the guest XML and be fairly happy. If they wanted to, they could explicitly include the extra features listed by capabilities XML, or just rely on the host-model. Then Spectre happened, and QEMU took the decision to almost double the number of x86 models, adding in -IBRS / -IBPB variants for most CPU model, so that applications could get the spec_ctrl / ibpb flags set without having to manually list them. In retrospect this was somewhat pointless, at least at the QEMU level, because there is little difference in complexity between the two approaches: -cpu Westmere,+spec-ctrl -cpu Westmere-IBRS At a higher level the extra named CPU models were slightly useful in so much as many application developers had taken a lazy approach and not provided users any way to explicitly turn on extra flags. This affected oVirt, OpenStack and virt-manager, and probably more. Though OpenStack since added ability to turn on arbitrary flags in response to the Spectre flaw, others have not. Then a recently along came the Speculative Store Bypass hardware vulnerability requiring addition of yet another CPU flag to guest configs. This required use of 'ssbd' on Intel and 'virt-ssbd' on AMD. While QEMU could have now added yet more CPU models, eg Westmere-SSBD, this does not feel like a winning strategy long term. Looking at the models how would a user have any clue whether the -IBRS or -SSBD or -NEXT-FLAW or -YET-ANOTHER-FLAW suffix is "better" ? So QEMU and libvirt took the joint decision to stop adding new named CPU models when CPU vulnerabilities are discovered from this point forwards. Applications / users would be expected to turn on CPU features explicitly as needed and are considered broken if they don't provide this functionality. As briefly mentioned above though, even before Spectre we had the pain of dealing with the -noTSX CPU models working around brokenness in the Intel TSX impl where they had to delete a CPU feature during microcode updates. This was rather painful to roll out at the time. An alternative to adding CPU models is to change meaning of existing CPU models. QEMU has a way todo this by tieing the change to machine types, and it has in fact been used to correct mistakes in the specification of CPU models in the past, when those mistakes have not had dependancies on microcode changes. This is not a particularly attractice way to deal with the errata. Short life distros tend to stick with upstream QEMU machine types and won't want to diverge by adding their own machine types. This gates them on having upstream define the extra machine types which is tricky under embargo. Long life distros do typically take on the burden of defining custom machine types, but usually only add them when doing major updates. The pain point with machine types is that the testing matrix grows at O(n^2) Using machine types for CPU security errata would significant increase the number of machine types and thus the testing matrix. eg if a security fix is needed in rhel-7.3, 7.4, 7.5 we can't just add a pc-rhel-7.5.1 machine with the fix, as it would not be possible to implement that in 7.3. So we would need would need pc-rhel-7.3.1, pc-rhel-7.4.1, pc-rhel-7.5.1, machine types, with 7.5 gaining all three. Finally CPU model changes have host hardware dependancies and machine types need to be independant of the host, since they are decided statically are build time. The only nice thing about machine type is that it is reasonably obvious what the "best" machine type is as they include a version number in the name, and users automatically get the best if they use an unversioned name. What if we can borrow the concept of versioning from machine types and apply it to CPU models directly. For example, considering the history of "Haswell" in QEMU, if we had versioned things, we would by now have: Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) Haswell-3.0.0 - added 'ssbd' (never done) If we followed the machine type approach, then a bare "Haswell" would statically resolve at build time to the most recent Haswell-X.X.X version associated with the QEMU release. This is unhelpful as we have a direct dependancy on the host hardware features. Better would be for a bare "Haswell" to be dynamically resolved at runtime, picking the most recent version that is capable of launching given the current hardware, KVM/TCG impl and QEMU version. ie -cpu Haswell should use Haswell-2.5.0 if on silicon with the TSX errata applied, but use Haswell-2.12.0 if the Spectre errata is applied in microcode, and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata. Versioning of CPU models as opposed to using arbitrary string suffixes (-noTSX, -IBRS) has a number of usability improvements that we would gain with versioned machine types, while avoiding exploding the machine type matrix. With versioned CPU models we can - Automatically tailor the best model based on hardware support - Users always get the best model if they use the bare CPU name - It is obvious to users which is the "best" / "newest" CPU model - Avoid combinatorial expansion of machines since same CPU model version can be added to all releases without adding machine types. - Users can still force a specific downgraded model by using the fully versioned name. Such versioning of CPU models would largely "just work" with existing libvirt versions, but to libvirt would really want to expand the bare CPU name to a versioned CPU name when recording new guest XML, so the ABI is preserved long term. An application like virt-manager which wants a simple UI can forever be happy simply giving users a list of bare CPU model names, and allowing libvirt / QEMU to automatically expand to the best versioned model for their host. An application like oVirt/OpenStack which wants direct control can allow the admin to choice if a bare name, or explicitly picking a versioned name if they need to cope with possibility of outdated hosts. 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 :|
* Daniel P. Berrangé (berrange@redhat.com) wrote:
This post is to raise question about helping use of named CPU models with KVM ie any case not using -cpu host.
In the old days (ie before 2018), the world was innocent and we had a nice set of named CPU models that corresponded to different Intel/AMD physical CPU families/generations (lets temporarily ignore the -noTSX fiasco).
An application could query libvirt to determine what the host CPU model was/is and use that model name in the guest XML and be fairly happy. If they wanted to, they could explicitly include the extra features listed by capabilities XML, or just rely on the host-model.
Then Spectre happened, and QEMU took the decision to almost double the number of x86 models, adding in -IBRS / -IBPB variants for most CPU model, so that applications could get the spec_ctrl / ibpb flags set without having to manually list them.
In retrospect this was somewhat pointless, at least at the QEMU level, because there is little difference in complexity between the two approaches:
-cpu Westmere,+spec-ctrl -cpu Westmere-IBRS
At a higher level the extra named CPU models were slightly useful in so much as many application developers had taken a lazy approach and not provided users any way to explicitly turn on extra flags. This affected oVirt, OpenStack and virt-manager, and probably more. Though OpenStack since added ability to turn on arbitrary flags in response to the Spectre flaw, others have not.
Then a recently along came the Speculative Store Bypass hardware vulnerability requiring addition of yet another CPU flag to guest configs. This required use of 'ssbd' on Intel and 'virt-ssbd' on AMD. While QEMU could have now added yet more CPU models, eg Westmere-SSBD, this does not feel like a winning strategy long term. Looking at the models how would a user have any clue whether the -IBRS or -SSBD or -NEXT-FLAW or -YET-ANOTHER-FLAW suffix is "better" ? So QEMU and libvirt took the joint decision to stop adding new named CPU models when CPU vulnerabilities are discovered from this point forwards. Applications / users would be expected to turn on CPU features explicitly as needed and are considered broken if they don't provide this functionality.
As briefly mentioned above though, even before Spectre we had the pain of dealing with the -noTSX CPU models working around brokenness in the Intel TSX impl where they had to delete a CPU feature during microcode updates. This was rather painful to roll out at the time.
An alternative to adding CPU models is to change meaning of existing CPU models. QEMU has a way todo this by tieing the change to machine types, and it has in fact been used to correct mistakes in the specification of CPU models in the past, when those mistakes have not had dependancies on microcode changes. This is not a particularly attractice way to deal with the errata. Short life distros tend to stick with upstream QEMU machine types and won't want to diverge by adding their own machine types. This gates them on having upstream define the extra machine types which is tricky under embargo. Long life distros do typically take on the burden of defining custom machine types, but usually only add them when doing major updates.
The pain point with machine types is that the testing matrix grows at O(n^2) Using machine types for CPU security errata would significant increase the number of machine types and thus the testing matrix. eg if a security fix is needed in rhel-7.3, 7.4, 7.5 we can't just add a pc-rhel-7.5.1 machine with the fix, as it would not be possible to implement that in 7.3. So we would need would need pc-rhel-7.3.1, pc-rhel-7.4.1, pc-rhel-7.5.1, machine types, with 7.5 gaining all three. Finally CPU model changes have host hardware dependancies and machine types need to be independant of the host, since they are decided statically are build time. The only nice thing about machine type is that it is reasonably obvious what the "best" machine type is as they include a version number in the name, and users automatically get the best if they use an unversioned name.
What if we can borrow the concept of versioning from machine types and apply it to CPU models directly. For example, considering the history of "Haswell" in QEMU, if we had versioned things, we would by now have:
Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) Haswell-3.0.0 - added 'ssbd' (never done)
OK. Note that this isn't that different to what happens on some real hardware where you have different 'steppings'
If we followed the machine type approach, then a bare "Haswell" would statically resolve at build time to the most recent Haswell-X.X.X version associated with the QEMU release. This is unhelpful as we have a direct dependancy on the host hardware features. Better would be for a bare "Haswell" to be dynamically resolved at runtime, picking the most recent version that is capable of launching given the current hardware, KVM/TCG impl and QEMU version.
ie -cpu Haswell
should use Haswell-2.5.0 if on silicon with the TSX errata applied, but use Haswell-2.12.0 if the Spectre errata is applied in microcode, and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata.
Versioning of CPU models as opposed to using arbitrary string suffixes (-noTSX, -IBRS) has a number of usability improvements that we would gain with versioned machine types, while avoiding exploding the machine type matrix. With versioned CPU models we can
- Automatically tailor the best model based on hardware support
- Users always get the best model if they use the bare CPU name
- It is obvious to users which is the "best" / "newest" CPU model
- Avoid combinatorial expansion of machines since same CPU model version can be added to all releases without adding machine types.
- Users can still force a specific downgraded model by using the fully versioned name.
Such versioning of CPU models would largely "just work" with existing libvirt versions, but to libvirt would really want to expand the bare CPU name to a versioned CPU name when recording new guest XML, so the ABI is preserved long term.
An application like virt-manager which wants a simple UI can forever be happy simply giving users a list of bare CPU model names, and allowing libvirt / QEMU to automatically expand to the best versioned model for their host.
An application like oVirt/OpenStack which wants direct control can allow the admin to choice if a bare name, or explicitly picking a versioned name if they need to cope with possibility of outdated hosts.
I fear people are going to find this out the hard way, when they add a new system into their cluster, a little bit later it gets a VM started on it, and then they try and migrate it to one of the older machines. Now if there was something that could take the CPU defintions from all the machines in the cluster and tell it which to use/which problems they had then that might make sense. It would be best for each higher level not to reinvent that. Would you restrict the combinations to cut down the test matrix - e.g. not allow Haswell-3.0.0 on anything prior to a 2.12 machine type? Dave
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 :|
-- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
On Thu, Jun 28, 2018 at 07:59:38PM +0100, Dr. David Alan Gilbert wrote: [...]
An application like virt-manager which wants a simple UI can forever be happy simply giving users a list of bare CPU model names, and allowing libvirt / QEMU to automatically expand to the best versioned model for their host.
An application like oVirt/OpenStack which wants direct control can allow the admin to choice if a bare name, or explicitly picking a versioned name if they need to cope with possibility of outdated hosts.
I fear people are going to find this out the hard way, when they add a new system into their cluster, a little bit later it gets a VM started on it, and then they try and migrate it to one of the older machines.
Now if there was something that could take the CPU defintions from all the machines in the cluster and tell it which to use/which problems they had then that might make sense. It would be best for each higher level not to reinvent that.
I think QEMU already provides enough info to allow that to be implemented. I'm not sure sure if the libvirt API already provides all the info needed for this (I think it does).
Would you restrict the combinations to cut down the test matrix - e.g. not allow Haswell-3.0.0 on anything prior to a 2.12 machine type?
Not sure if it would be worth the extra complexity: we would need an interface to tell libvirt which CPU models are usable on which machine-types. But a generic mechanism to tell libvirt which devices are allowed on each machine-type would very interesting to have, anyway. -- Eduardo
On Thu, Jun 28, 2018 at 16:23:53 -0300, Eduardo Habkost wrote:
On Thu, Jun 28, 2018 at 07:59:38PM +0100, Dr. David Alan Gilbert wrote: [...]
An application like virt-manager which wants a simple UI can forever be happy simply giving users a list of bare CPU model names, and allowing libvirt / QEMU to automatically expand to the best versioned model for their host.
An application like oVirt/OpenStack which wants direct control can allow the admin to choice if a bare name, or explicitly picking a versioned name if they need to cope with possibility of outdated hosts.
I fear people are going to find this out the hard way, when they add a new system into their cluster, a little bit later it gets a VM started on it, and then they try and migrate it to one of the older machines.
Now if there was something that could take the CPU defintions from all the machines in the cluster and tell it which to use/which problems they had then that might make sense. It would be best for each higher level not to reinvent that.
I think QEMU already provides enough info to allow that to be implemented. I'm not sure sure if the libvirt API already provides all the info needed for this (I think it does).
Right, libvirt provides virConnectBaselineHypervisorCPU API which accepts a list of host CPU models from several hosts and returns the best CPU definition runnable on these hosts. However, OpenStack clusters are too dynamic for this to be practical so the admin would need to take care of this by setting an appropriate model statically.
Would you restrict the combinations to cut down the test matrix - e.g. not allow Haswell-3.0.0 on anything prior to a 2.12 machine type?
Not sure if it would be worth the extra complexity: we would need an interface to tell libvirt which CPU models are usable on which machine-types.
In case we do this, libvirt should already by ready for it on the API level for both reporting capabilities and CPU comparison/baseline. All these APIs already accept machine type as an optional parameter so that different results can be provided depending on machine type. Jirka
On Fri, Jun 29, 2018 at 08:06:04AM +0200, Jiri Denemark wrote:
On Thu, Jun 28, 2018 at 16:23:53 -0300, Eduardo Habkost wrote:
On Thu, Jun 28, 2018 at 07:59:38PM +0100, Dr. David Alan Gilbert wrote: [...]
Would you restrict the combinations to cut down the test matrix - e.g. not allow Haswell-3.0.0 on anything prior to a 2.12 machine type?
Not sure if it would be worth the extra complexity: we would need an interface to tell libvirt which CPU models are usable on which machine-types.
In case we do this, libvirt should already by ready for it on the API level for both reporting capabilities and CPU comparison/baseline. All these APIs already accept machine type as an optional parameter so that different results can be provided depending on machine type.
Does it have an abstraction that can already represent "CPU model $C can/can't be used with machine-type $M"? -- Eduardo
On Thu, Jun 28, 2018 at 07:59:38PM +0100, Dr. David Alan Gilbert wrote:
* Daniel P. Berrangé (berrange@redhat.com) wrote:
This post is to raise question about helping use of named CPU models with KVM ie any case not using -cpu host.
In the old days (ie before 2018), the world was innocent and we had a nice set of named CPU models that corresponded to different Intel/AMD physical CPU families/generations (lets temporarily ignore the -noTSX fiasco).
An application could query libvirt to determine what the host CPU model was/is and use that model name in the guest XML and be fairly happy. If they wanted to, they could explicitly include the extra features listed by capabilities XML, or just rely on the host-model.
Then Spectre happened, and QEMU took the decision to almost double the number of x86 models, adding in -IBRS / -IBPB variants for most CPU model, so that applications could get the spec_ctrl / ibpb flags set without having to manually list them.
In retrospect this was somewhat pointless, at least at the QEMU level, because there is little difference in complexity between the two approaches:
-cpu Westmere,+spec-ctrl -cpu Westmere-IBRS
At a higher level the extra named CPU models were slightly useful in so much as many application developers had taken a lazy approach and not provided users any way to explicitly turn on extra flags. This affected oVirt, OpenStack and virt-manager, and probably more. Though OpenStack since added ability to turn on arbitrary flags in response to the Spectre flaw, others have not.
Then a recently along came the Speculative Store Bypass hardware vulnerability requiring addition of yet another CPU flag to guest configs. This required use of 'ssbd' on Intel and 'virt-ssbd' on AMD. While QEMU could have now added yet more CPU models, eg Westmere-SSBD, this does not feel like a winning strategy long term. Looking at the models how would a user have any clue whether the -IBRS or -SSBD or -NEXT-FLAW or -YET-ANOTHER-FLAW suffix is "better" ? So QEMU and libvirt took the joint decision to stop adding new named CPU models when CPU vulnerabilities are discovered from this point forwards. Applications / users would be expected to turn on CPU features explicitly as needed and are considered broken if they don't provide this functionality.
As briefly mentioned above though, even before Spectre we had the pain of dealing with the -noTSX CPU models working around brokenness in the Intel TSX impl where they had to delete a CPU feature during microcode updates. This was rather painful to roll out at the time.
An alternative to adding CPU models is to change meaning of existing CPU models. QEMU has a way todo this by tieing the change to machine types, and it has in fact been used to correct mistakes in the specification of CPU models in the past, when those mistakes have not had dependancies on microcode changes. This is not a particularly attractice way to deal with the errata. Short life distros tend to stick with upstream QEMU machine types and won't want to diverge by adding their own machine types. This gates them on having upstream define the extra machine types which is tricky under embargo. Long life distros do typically take on the burden of defining custom machine types, but usually only add them when doing major updates.
The pain point with machine types is that the testing matrix grows at O(n^2) Using machine types for CPU security errata would significant increase the number of machine types and thus the testing matrix. eg if a security fix is needed in rhel-7.3, 7.4, 7.5 we can't just add a pc-rhel-7.5.1 machine with the fix, as it would not be possible to implement that in 7.3. So we would need would need pc-rhel-7.3.1, pc-rhel-7.4.1, pc-rhel-7.5.1, machine types, with 7.5 gaining all three. Finally CPU model changes have host hardware dependancies and machine types need to be independant of the host, since they are decided statically are build time. The only nice thing about machine type is that it is reasonably obvious what the "best" machine type is as they include a version number in the name, and users automatically get the best if they use an unversioned name.
What if we can borrow the concept of versioning from machine types and apply it to CPU models directly. For example, considering the history of "Haswell" in QEMU, if we had versioned things, we would by now have:
Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) Haswell-3.0.0 - added 'ssbd' (never done)
OK. Note that this isn't that different to what happens on some real hardware where you have different 'steppings'
If we followed the machine type approach, then a bare "Haswell" would statically resolve at build time to the most recent Haswell-X.X.X version associated with the QEMU release. This is unhelpful as we have a direct dependancy on the host hardware features. Better would be for a bare "Haswell" to be dynamically resolved at runtime, picking the most recent version that is capable of launching given the current hardware, KVM/TCG impl and QEMU version.
ie -cpu Haswell
should use Haswell-2.5.0 if on silicon with the TSX errata applied, but use Haswell-2.12.0 if the Spectre errata is applied in microcode, and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata.
Versioning of CPU models as opposed to using arbitrary string suffixes (-noTSX, -IBRS) has a number of usability improvements that we would gain with versioned machine types, while avoiding exploding the machine type matrix. With versioned CPU models we can
- Automatically tailor the best model based on hardware support
- Users always get the best model if they use the bare CPU name
- It is obvious to users which is the "best" / "newest" CPU model
- Avoid combinatorial expansion of machines since same CPU model version can be added to all releases without adding machine types.
- Users can still force a specific downgraded model by using the fully versioned name.
Such versioning of CPU models would largely "just work" with existing libvirt versions, but to libvirt would really want to expand the bare CPU name to a versioned CPU name when recording new guest XML, so the ABI is preserved long term.
An application like virt-manager which wants a simple UI can forever be happy simply giving users a list of bare CPU model names, and allowing libvirt / QEMU to automatically expand to the best versioned model for their host.
An application like oVirt/OpenStack which wants direct control can allow the admin to choice if a bare name, or explicitly picking a versioned name if they need to cope with possibility of outdated hosts.
I fear people are going to find this out the hard way, when they add a new system into their cluster, a little bit later it gets a VM started on it, and then they try and migrate it to one of the older machines.
Now if there was something that could take the CPU defintions from all the machines in the cluster and tell it which to use/which problems they had then that might make sense. It would be best for each higher level not to reinvent that.
Libvirt / QEMU have ability to let apps do that. Nova could choose to do that, but it does not wish to take advantage of that right now. The nova admin already has make an explicit config to set the maximum Nova database object schema version, and maximum machine type version across the cluster. The max CPU model version fits in with what they are already doing quite well. Essentially the admin knows what versions of QEMU they have across their cluster - generally it is jus N, and N+1 during course of a version upgrade. So it is quite easy for the admin to know what the max version should be. In fact the admin doens't really need to be involved in it at all - the cluster provisioning tool already knows the old + new host versions, so can set it automatically at the right times during upgrade process.
Would you restrict the combinations to cut down the test matrix - e.g. not allow Haswell-3.0.0 on anything prior to a 2.12 machine type?
The key point of having CPU versions separate from machine type versions, is to allow arbitrary mixing of versions. For example when we issued Spectre updates for RHEL, we had to add new CPU model to something like 12 different QEMU versions, and we needed the same CPU model changes to work with all the different machine types we had across those versions. So restricting CPU versions to only certain machine types would defeat the puporse of having CPU versions. 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 28, 2018 at 04:45:02PM +0100, Daniel P. Berrangé wrote: [...]
What if we can borrow the concept of versioning from machine types and apply it to CPU models directly. For example, considering the history of "Haswell" in QEMU, if we had versioned things, we would by now have:
Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) Haswell-3.0.0 - added 'ssbd' (never done)
If we followed the machine type approach, then a bare "Haswell" would statically resolve at build time to the most recent Haswell-X.X.X version associated with the QEMU release. This is unhelpful as we have a direct dependancy on the host hardware features. Better would be for a bare "Haswell" to be dynamically resolved at runtime, picking the most recent version that is capable of launching given the current hardware, KVM/TCG impl and QEMU version.
ie -cpu Haswell
should use Haswell-2.5.0 if on silicon with the TSX errata applied, but use Haswell-2.12.0 if the Spectre errata is applied in microcode, and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata.
Doing this unconditionally would make "-machine pc-q35-3.1 -cpu Haswell" unsafe for live migration, and break existing usage. But this behavior could be enabled explicitly somehow.
Versioning of CPU models as opposed to using arbitrary string suffixes (-noTSX, -IBRS) has a number of usability improvements that we would gain with versioned machine types, while avoiding exploding the machine type matrix. With versioned CPU models we can
- Automatically tailor the best model based on hardware support
- Users always get the best model if they use the bare CPU name
- It is obvious to users which is the "best" / "newest" CPU model
- Avoid combinatorial expansion of machines since same CPU model version can be added to all releases without adding machine types.
- Users can still force a specific downgraded model by using the fully versioned name.
Such versioning of CPU models would largely "just work" with existing libvirt versions, but to libvirt would really want to expand the bare CPU name to a versioned CPU name when recording new guest XML, so the ABI is preserved long term.
An application like virt-manager which wants a simple UI can forever be happy simply giving users a list of bare CPU model names, and allowing libvirt / QEMU to automatically expand to the best versioned model for their host.
An application like oVirt/OpenStack which wants direct control can allow the admin to choice if a bare name, or explicitly picking a versioned name if they need to cope with possibility of outdated hosts.
The proposal makes sense, and I think most of it can be already implemented on top of existing query-cpu-model-* commands. query-cpu-model-expansion type=static can expand to a versioned CPU model. We will probably need to make query-cpu-model-expansion accept a machine-type name as input, and/or add a new flag meaning "please give me the best CPU version you have, not the one defined by the current machine-type". I'm not sure what would be the best way to encode two types of information, though: * Fallback/alternatives info, e.g.: "It makes sense to use Haswell-{3.0,2.12,2.5,...} if Haswell-3.1 is not runnable and the user asked for Haswell". * Ordering/preference info, e.g.: "Haswell-3.1 is better than Haswell-3.0, prefer the latter" -- Eduardo
* Eduardo Habkost (ehabkost@redhat.com) wrote:
On Thu, Jun 28, 2018 at 04:45:02PM +0100, Daniel P. Berrangé wrote: [...]
What if we can borrow the concept of versioning from machine types and apply it to CPU models directly. For example, considering the history of "Haswell" in QEMU, if we had versioned things, we would by now have:
Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) Haswell-3.0.0 - added 'ssbd' (never done)
If we followed the machine type approach, then a bare "Haswell" would statically resolve at build time to the most recent Haswell-X.X.X version associated with the QEMU release. This is unhelpful as we have a direct dependancy on the host hardware features. Better would be for a bare "Haswell" to be dynamically resolved at runtime, picking the most recent version that is capable of launching given the current hardware, KVM/TCG impl and QEMU version.
ie -cpu Haswell
should use Haswell-2.5.0 if on silicon with the TSX errata applied, but use Haswell-2.12.0 if the Spectre errata is applied in microcode, and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata.
Doing this unconditionally would make "-machine pc-q35-3.1 -cpu Haswell" unsafe for live migration, and break existing usage. But this behavior could be enabled explicitly somehow.
Versioning of CPU models as opposed to using arbitrary string suffixes (-noTSX, -IBRS) has a number of usability improvements that we would gain with versioned machine types, while avoiding exploding the machine type matrix. With versioned CPU models we can
- Automatically tailor the best model based on hardware support
- Users always get the best model if they use the bare CPU name
- It is obvious to users which is the "best" / "newest" CPU model
- Avoid combinatorial expansion of machines since same CPU model version can be added to all releases without adding machine types.
- Users can still force a specific downgraded model by using the fully versioned name.
Such versioning of CPU models would largely "just work" with existing libvirt versions, but to libvirt would really want to expand the bare CPU name to a versioned CPU name when recording new guest XML, so the ABI is preserved long term.
An application like virt-manager which wants a simple UI can forever be happy simply giving users a list of bare CPU model names, and allowing libvirt / QEMU to automatically expand to the best versioned model for their host.
An application like oVirt/OpenStack which wants direct control can allow the admin to choice if a bare name, or explicitly picking a versioned name if they need to cope with possibility of outdated hosts.
The proposal makes sense, and I think most of it can be already implemented on top of existing query-cpu-model-* commands. query-cpu-model-expansion type=static can expand to a versioned CPU model.
We will probably need to make query-cpu-model-expansion accept a machine-type name as input, and/or add a new flag meaning "please give me the best CPU version you have, not the one defined by the current machine-type".
I'm not sure what would be the best way to encode two types of information, though:
Both of those are solved with the numbering scheme
* Fallback/alternatives info, e.g.: "It makes sense to use Haswell-{3.0,2.12,2.5,...} if Haswell-3.1 is not runnable and the user asked for Haswell".
Use the highest that works.
* Ordering/preference info, e.g.: "Haswell-3.1 is better than Haswell-3.0, prefer the latter"
Higher is better. The only thing that worries me about a numbering scheme is that it's now more difficult for a user to know whether they've got the type with a fix for a particular vulnerability. We're going to have to say something like: 'For the new XYZ vulnerability make sure you're using Haswell-3.2 or later, SkyLake-2.6 or later, Westmere-4.8 or later .....' which all gets a bit confusing. Dave
-- Eduardo
-- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
On Fri, Jun 29, 2018 at 09:53:53AM +0100, Dr. David Alan Gilbert wrote:
* Eduardo Habkost (ehabkost@redhat.com) wrote:
On Thu, Jun 28, 2018 at 04:45:02PM +0100, Daniel P. Berrangé wrote: [...]
What if we can borrow the concept of versioning from machine types and apply it to CPU models directly. For example, considering the history of "Haswell" in QEMU, if we had versioned things, we would by now have:
Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) Haswell-3.0.0 - added 'ssbd' (never done)
If we followed the machine type approach, then a bare "Haswell" would statically resolve at build time to the most recent Haswell-X.X.X version associated with the QEMU release. This is unhelpful as we have a direct dependancy on the host hardware features. Better would be for a bare "Haswell" to be dynamically resolved at runtime, picking the most recent version that is capable of launching given the current hardware, KVM/TCG impl and QEMU version.
ie -cpu Haswell
should use Haswell-2.5.0 if on silicon with the TSX errata applied, but use Haswell-2.12.0 if the Spectre errata is applied in microcode, and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata.
Doing this unconditionally would make "-machine pc-q35-3.1 -cpu Haswell" unsafe for live migration, and break existing usage. But this behavior could be enabled explicitly somehow.
Versioning of CPU models as opposed to using arbitrary string suffixes (-noTSX, -IBRS) has a number of usability improvements that we would gain with versioned machine types, while avoiding exploding the machine type matrix. With versioned CPU models we can
- Automatically tailor the best model based on hardware support
- Users always get the best model if they use the bare CPU name
- It is obvious to users which is the "best" / "newest" CPU model
- Avoid combinatorial expansion of machines since same CPU model version can be added to all releases without adding machine types.
- Users can still force a specific downgraded model by using the fully versioned name.
Such versioning of CPU models would largely "just work" with existing libvirt versions, but to libvirt would really want to expand the bare CPU name to a versioned CPU name when recording new guest XML, so the ABI is preserved long term.
An application like virt-manager which wants a simple UI can forever be happy simply giving users a list of bare CPU model names, and allowing libvirt / QEMU to automatically expand to the best versioned model for their host.
An application like oVirt/OpenStack which wants direct control can allow the admin to choice if a bare name, or explicitly picking a versioned name if they need to cope with possibility of outdated hosts.
The proposal makes sense, and I think most of it can be already implemented on top of existing query-cpu-model-* commands. query-cpu-model-expansion type=static can expand to a versioned CPU model.
We will probably need to make query-cpu-model-expansion accept a machine-type name as input, and/or add a new flag meaning "please give me the best CPU version you have, not the one defined by the current machine-type".
I'm not sure what would be the best way to encode two types of information, though:
Both of those are solved with the numbering scheme
* Fallback/alternatives info, e.g.: "It makes sense to use Haswell-{3.0,2.12,2.5,...} if Haswell-3.1 is not runnable and the user asked for Haswell".
Use the highest that works.
* Ordering/preference info, e.g.: "Haswell-3.1 is better than Haswell-3.0, prefer the latter"
Higher is better.
The only thing that worries me about a numbering scheme is that it's now more difficult for a user to know whether they've got the type with a fix for a particular vulnerability.
True, but if more vulns arrive we have the same problem with named suffixes too. eg if we added -SSBD variants, users would ask whether -SSBD includes the -IBRS fix or vica-verca, as a year down the line they're not going to remember which or SSBD/IBRS came out first.
We're going to have to say something like: 'For the new XYZ vulnerability make sure you're using Haswell-3.2 or later, SkyLake-2.6 or later, Westmere-4.8 or later .....'
which all gets a bit confusing.
The kernel has a /sys/devices/system/cpu/vulnerabilities dir that lists status of various flaws. I have been thinking about whether libvirt should create a 'virt-guest-validate' command that looks at guest XML and reports whether any of the config settings are vulnerable or otherwise diverging from best practice in some way. QEMU itself would perhaps have a 'query-vulnerabilities' monitor command to report whether the current config is satisfactory or not. Ultimately though, getting a fixed guest involves host kernel, microcode, qemu, and guest kernel. So to get a true picture of your safety people should really look straight to the guest kernels' /sys/devices/system/cpu/vulnerabilities directory. They only need to look at host/microcode/qemu if the guest is reporting something is wrong. 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 Fri, Jun 29, 2018 at 11:19:17AM +0100, Daniel P. Berrangé wrote:
On Fri, Jun 29, 2018 at 09:53:53AM +0100, Dr. David Alan Gilbert wrote: [...]
We're going to have to say something like: 'For the new XYZ vulnerability make sure you're using Haswell-3.2 or later, SkyLake-2.6 or later, Westmere-4.8 or later .....'
which all gets a bit confusing.
The kernel has a /sys/devices/system/cpu/vulnerabilities dir that lists status of various flaws.
I have been thinking about whether libvirt should create a 'virt-guest-validate' command that looks at guest XML and reports whether any of the config settings are vulnerable or otherwise diverging from best practice in some way.
QEMU itself would perhaps have a 'query-vulnerabilities' monitor command to report whether the current config is satisfactory or not.
Makes sense to me. I wanted to make QEMU emit warnings on obviously insecure configurations. Adding a query-vulnerabilities command would be the QMP counterpart of that. -- Eduardo
On Thu, Jun 28, 2018 at 04:52:27PM -0300, Eduardo Habkost wrote:
On Thu, Jun 28, 2018 at 04:45:02PM +0100, Daniel P. Berrangé wrote: [...]
What if we can borrow the concept of versioning from machine types and apply it to CPU models directly. For example, considering the history of "Haswell" in QEMU, if we had versioned things, we would by now have:
Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) Haswell-3.0.0 - added 'ssbd' (never done)
If we followed the machine type approach, then a bare "Haswell" would statically resolve at build time to the most recent Haswell-X.X.X version associated with the QEMU release. This is unhelpful as we have a direct dependancy on the host hardware features. Better would be for a bare "Haswell" to be dynamically resolved at runtime, picking the most recent version that is capable of launching given the current hardware, KVM/TCG impl and QEMU version.
ie -cpu Haswell
should use Haswell-2.5.0 if on silicon with the TSX errata applied, but use Haswell-2.12.0 if the Spectre errata is applied in microcode, and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata.
Doing this unconditionally would make "-machine pc-q35-3.1 -cpu Haswell" unsafe for live migration, and break existing usage. But this behavior could be enabled explicitly somehow.
True, for full back compat with existing libvirt we would probably want to opt-in to it. eg -cpu Haswell could pick a fixed Haswell--XXX version according to the machine type. -cpu Haswell,best=on could pick best version for the host with the caveat about migration between heterogenous hosts.
Versioning of CPU models as opposed to using arbitrary string suffixes (-noTSX, -IBRS) has a number of usability improvements that we would gain with versioned machine types, while avoiding exploding the machine type matrix. With versioned CPU models we can
- Automatically tailor the best model based on hardware support
- Users always get the best model if they use the bare CPU name
- It is obvious to users which is the "best" / "newest" CPU model
- Avoid combinatorial expansion of machines since same CPU model version can be added to all releases without adding machine types.
- Users can still force a specific downgraded model by using the fully versioned name.
Such versioning of CPU models would largely "just work" with existing libvirt versions, but to libvirt would really want to expand the bare CPU name to a versioned CPU name when recording new guest XML, so the ABI is preserved long term.
An application like virt-manager which wants a simple UI can forever be happy simply giving users a list of bare CPU model names, and allowing libvirt / QEMU to automatically expand to the best versioned model for their host.
An application like oVirt/OpenStack which wants direct control can allow the admin to choice if a bare name, or explicitly picking a versioned name if they need to cope with possibility of outdated hosts.
The proposal makes sense, and I think most of it can be already implemented on top of existing query-cpu-model-* commands. query-cpu-model-expansion type=static can expand to a versioned CPU model.
We will probably need to make query-cpu-model-expansion accept a machine-type name as input, and/or add a new flag meaning "please give me the best CPU version you have, not the one defined by the current machine-type".
I'm not sure what would be the best way to encode two types of information, though:
* Fallback/alternatives info, e.g.: "It makes sense to use Haswell-{3.0,2.12,2.5,...} if Haswell-3.1 is not runnable and the user asked for Haswell".
* Ordering/preference info, e.g.: "Haswell-3.1 is better than Haswell-3.0, prefer the latter"
The version number of course gives an ordering, but we generally tell people not to assume version is numeric. We could report an explicit "priority" in some manner against each. 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 Fri, Jun 29, 2018 at 11:14:17 +0100, Daniel P. Berrangé wrote:
On Thu, Jun 28, 2018 at 04:52:27PM -0300, Eduardo Habkost wrote:
On Thu, Jun 28, 2018 at 04:45:02PM +0100, Daniel P. Berrangé wrote: [...]
What if we can borrow the concept of versioning from machine types and apply it to CPU models directly. For example, considering the history of "Haswell" in QEMU, if we had versioned things, we would by now have:
Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) Haswell-3.0.0 - added 'ssbd' (never done)
If we followed the machine type approach, then a bare "Haswell" would statically resolve at build time to the most recent Haswell-X.X.X version associated with the QEMU release. This is unhelpful as we have a direct dependancy on the host hardware features. Better would be for a bare "Haswell" to be dynamically resolved at runtime, picking the most recent version that is capable of launching given the current hardware, KVM/TCG impl and QEMU version.
ie -cpu Haswell
should use Haswell-2.5.0 if on silicon with the TSX errata applied, but use Haswell-2.12.0 if the Spectre errata is applied in microcode, and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata.
Doing this unconditionally would make "-machine pc-q35-3.1 -cpu Haswell" unsafe for live migration, and break existing usage. But this behavior could be enabled explicitly somehow.
True, for full back compat with existing libvirt we would probably want to opt-in to it.
eg -cpu Haswell could pick a fixed Haswell--XXX version according to the machine type. -cpu Haswell,best=on could pick best version for the host with the caveat about migration between heterogenous hosts.
I was thinking we could even separate the CPU model version from the name itself: -cpu Haswell (the old, compatible way) -cpu Haswell,version=best -cpu Haswell,version=2.12.0 It would be slightly more work for the upper management layers, but IMHO it would make more sense. In any case, we have to think about keeping guest ABI stable. I hope the automatic version selection would not cause any problems for subsequent cold starts (such as Windows activation issues). It should be very similar to updating CPU microcode which the guest OS is already supposed to deal with in real hardware. However, in the past QEMU changed CPU signature (family, model, stepping) for new machine types and it is likely to happen with separately versioned CPU models too. I believe CPU microcode updates do not touch these values. On the other hand, it's similar to host-model and the user can always specify exact version to avoid this slight change should it be a problem. Once the domain starts, we need to keep stable ABI across migrations, save/restores, or snapshots. Libvirt already does so by talking to QEMU before starting vCPUs and checking for disabled/enabled features. Then we store this information in the active domain XML to make sure we can enforce the same CPU later. This concept would need to be enhanced to include the CPU model version which QEMU would need to be able to report. A significantly more fun would result from letting libvirt use the versioned CPU model stuff by default without an explicit knob in the XML. But I guess you don't want to go that direction, do you? Jirka
On Fri, Jun 29, 2018 at 02:12:51PM +0200, Jiri Denemark wrote:
On Fri, Jun 29, 2018 at 11:14:17 +0100, Daniel P. Berrangé wrote:
On Thu, Jun 28, 2018 at 04:52:27PM -0300, Eduardo Habkost wrote:
On Thu, Jun 28, 2018 at 04:45:02PM +0100, Daniel P. Berrangé wrote: [...]
What if we can borrow the concept of versioning from machine types and apply it to CPU models directly. For example, considering the history of "Haswell" in QEMU, if we had versioned things, we would by now have:
Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) Haswell-3.0.0 - added 'ssbd' (never done)
If we followed the machine type approach, then a bare "Haswell" would statically resolve at build time to the most recent Haswell-X.X.X version associated with the QEMU release. This is unhelpful as we have a direct dependancy on the host hardware features. Better would be for a bare "Haswell" to be dynamically resolved at runtime, picking the most recent version that is capable of launching given the current hardware, KVM/TCG impl and QEMU version.
ie -cpu Haswell
should use Haswell-2.5.0 if on silicon with the TSX errata applied, but use Haswell-2.12.0 if the Spectre errata is applied in microcode, and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata.
Doing this unconditionally would make "-machine pc-q35-3.1 -cpu Haswell" unsafe for live migration, and break existing usage. But this behavior could be enabled explicitly somehow.
True, for full back compat with existing libvirt we would probably want to opt-in to it.
eg -cpu Haswell could pick a fixed Haswell--XXX version according to the machine type. -cpu Haswell,best=on could pick best version for the host with the caveat about migration between heterogenous hosts.
I was thinking we could even separate the CPU model version from the name itself:
-cpu Haswell (the old, compatible way) -cpu Haswell,version=best -cpu Haswell,version=2.12.0
It would be slightly more work for the upper management layers, but IMHO it would make more sense.
In any case, we have to think about keeping guest ABI stable.
I hope the automatic version selection would not cause any problems for subsequent cold starts (such as Windows activation issues). It should be very similar to updating CPU microcode which the guest OS is already supposed to deal with in real hardware. However, in the past QEMU changed CPU signature (family, model, stepping) for new machine types and it is likely to happen with separately versioned CPU models too. I believe CPU microcode updates do not touch these values. On the other hand, it's similar to host-model and the user can always specify exact version to avoid this slight change should it be a problem.
Once the domain starts, we need to keep stable ABI across migrations, save/restores, or snapshots. Libvirt already does so by talking to QEMU before starting vCPUs and checking for disabled/enabled features. Then we store this information in the active domain XML to make sure we can enforce the same CPU later. This concept would need to be enhanced to include the CPU model version which QEMU would need to be able to report.
A significantly more fun would result from letting libvirt use the versioned CPU model stuff by default without an explicit knob in the XML. But I guess you don't want to go that direction, do you?
From libvirt POV I think we have to always fully expand the version at time of define, in the same way we expand the machine type. Nothing else can guarantee ABI.
So once you do have a guest and you want to update it to use a newer version, you'd have to explicitly delete the version from the XML and let libvirt update it again, similar to how you update existing guest to new machine type. We could perhaps make this easier - eg VIR_DOMAIN_XML_UPDATE_CPU flag for virDomainDefine/virDomainCreateXML or similar. 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 Fri, Jun 29, 2018 at 02:12:51PM +0200, Jiri Denemark wrote:
On Fri, Jun 29, 2018 at 11:14:17 +0100, Daniel P. Berrangé wrote:
On Thu, Jun 28, 2018 at 04:52:27PM -0300, Eduardo Habkost wrote:
On Thu, Jun 28, 2018 at 04:45:02PM +0100, Daniel P. Berrangé wrote: [...]
What if we can borrow the concept of versioning from machine types and apply it to CPU models directly. For example, considering the history of "Haswell" in QEMU, if we had versioned things, we would by now have:
Haswell-1.3.0 - first version (37507094f350b75c62dc059f998e7185de3ab60a) Haswell-2.2.0 - added 'rdrand' (78a611f1936b3eac8ed78a2be2146a742a85212c_ Haswell-2.3.0 - removed 'hle' & 'rtm' (a356850b80b3d13b2ef737dad2acb05e6da03753) Haswell-2.5.0 - added 'abm' (becb66673ec30cb604926d247ab9449a60ad8b11 Haswell-2.12.0 - added 'spec-ctrl' (ac96c41354b7e4c70b756342d9b686e31ab87458) Haswell-3.0.0 - added 'ssbd' (never done)
If we followed the machine type approach, then a bare "Haswell" would statically resolve at build time to the most recent Haswell-X.X.X version associated with the QEMU release. This is unhelpful as we have a direct dependancy on the host hardware features. Better would be for a bare "Haswell" to be dynamically resolved at runtime, picking the most recent version that is capable of launching given the current hardware, KVM/TCG impl and QEMU version.
ie -cpu Haswell
should use Haswell-2.5.0 if on silicon with the TSX errata applied, but use Haswell-2.12.0 if the Spectre errata is applied in microcode, and use Haswell-3.0.0 once Intel finally releases SSBD microcode errata.
Doing this unconditionally would make "-machine pc-q35-3.1 -cpu Haswell" unsafe for live migration, and break existing usage. But this behavior could be enabled explicitly somehow.
True, for full back compat with existing libvirt we would probably want to opt-in to it.
eg -cpu Haswell could pick a fixed Haswell--XXX version according to the machine type. -cpu Haswell,best=on could pick best version for the host with the caveat about migration between heterogenous hosts.
I was thinking we could even separate the CPU model version from the name itself:
-cpu Haswell (the old, compatible way) -cpu Haswell,version=best -cpu Haswell,version=2.12.0
That's a nice idea. The only problem I see is that this: -> query-cpu-model-expansion type=static model=Haswell <- { model: { name: "Haswell-2.12.0" } } is returning a static CPU model ("Haswell-2.12.0") on `model.name`, which matches the documentation for type=static. But this: -> query-cpu-model-expansion type=static model=Haswell <- { model: { name: "Haswell", version="2.12.0" } } is returning a non-static CPU model name ("Haswell") on `model.name`, which breaks the existing documentation of type=static ("Expand to a static CPU model, a combination of a static base model name and property delta changes"). Maybe this would work: -> query-cpu-model-expansion type=static model=Haswell <- { model: { name: "Haswell-base", version="2.12.0" } } "Haswell-base" would be a static CPU model. "Haswell" would be a non-static but migration-safe CPU model (which is already the case today). Having a "Haswell-2.12.0" alias (that looks like a regular CPU model) for legacy management management software would be possible too. -- Eduardo
On Fri, Jun 29, 2018 at 11:14:17AM +0100, Daniel P. Berrangé wrote:
On Thu, Jun 28, 2018 at 04:52:27PM -0300, Eduardo Habkost wrote: [...]
I'm not sure what would be the best way to encode two types of information, though:
* Fallback/alternatives info, e.g.: "It makes sense to use Haswell-{3.0,2.12,2.5,...} if Haswell-3.1 is not runnable and the user asked for Haswell".
* Ordering/preference info, e.g.: "Haswell-3.1 is better than Haswell-3.0, prefer the latter"
The version number of course gives an ordering, but we generally tell people not to assume version is numeric. We could report an explicit "priority" in some manner against each.
Makes sense. "priority" could be included on query-cpu-definitions to help software choose the best alternative, and "version" could be just an opaque string that libvirt needs to save after expanding a CPU model. -- Eduardo
participants (4)
-
Daniel P. Berrangé -
Dr. David Alan Gilbert -
Eduardo Habkost -
Jiri Denemark