| Message ID | 20210401151957.408028-1-vkuznets@redhat.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | i386: Fix interrupt based Async PF enablement | expand |
On 01/04/21 17:19, Vitaly Kuznetsov wrote: > I noticed two issues with 'kvm-asyncpf-int' enablement: > 1) We forgot to add to to kvm_default_props[] so it doesn't get enabled > automatically (unless '-cpu host' is used or the feature is enabled > manually on the command line) > 2) We forgot to disable it for older machine types to preserve migration. > This went unnoticed because of 1) I believe. > > Vitaly Kuznetsov (2): > i386: Add 'kvm-asyncpf-int' to kvm_default_props array > i386: Disable 'kvm-asyncpf-int' feature for machine types <= 5.1 > > hw/i386/pc.c | 1 + > target/i386/cpu.c | 1 + > 2 files changed, 2 insertions(+) > Wasn't this intentional to avoid requiring a new kernel version? Paolo
Paolo Bonzini <pbonzini@redhat.com> writes: > On 01/04/21 17:19, Vitaly Kuznetsov wrote: >> I noticed two issues with 'kvm-asyncpf-int' enablement: >> 1) We forgot to add to to kvm_default_props[] so it doesn't get enabled >> automatically (unless '-cpu host' is used or the feature is enabled >> manually on the command line) >> 2) We forgot to disable it for older machine types to preserve migration. >> This went unnoticed because of 1) I believe. >> >> Vitaly Kuznetsov (2): >> i386: Add 'kvm-asyncpf-int' to kvm_default_props array >> i386: Disable 'kvm-asyncpf-int' feature for machine types <= 5.1 >> >> hw/i386/pc.c | 1 + >> target/i386/cpu.c | 1 + >> 2 files changed, 2 insertions(+) >> > > Wasn't this intentional to avoid requiring a new kernel version? I think I forgot the initial plan :-( The problem is that after we disabled the original APF (#PF based) almost nobody is using the feature as it needs to be enabled explicitly on the command line. Several considerations regarding the default: if your kernel doesn't support the feature you get as much as a warning: qemu-system-x86_64: warning: host doesn't support requested feature: CPUID.40000001H:EAX.kvm-asyncpf-int [bit 14] older machine types are still available (I disable it for <= 5.1 but we can consider disabling it for 5.2 too). The feature is upstream since Linux 5.8, I know that QEMU supports much older kernels but this doesn't probably mean that we can't enable new KVM PV features unless all supported kernels have it, we'd have to wait many years otherwise.
On 06/04/21 13:42, Vitaly Kuznetsov wrote: > older machine types are still available (I disable it for <= 5.1 but we > can consider disabling it for 5.2 too). The feature is upstream since > Linux 5.8, I know that QEMU supports much older kernels but this doesn't > probably mean that we can't enable new KVM PV features unless all > supported kernels have it, we'd have to wait many years otherwise. Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, though that will go away in 6.1. We should take the occasion of dropping RHEL7 to be clearer about which kernels are supported. Paolo
* Paolo Bonzini (pbonzini@redhat.com) wrote: > On 06/04/21 13:42, Vitaly Kuznetsov wrote: > > older machine types are still available (I disable it for <= 5.1 but we > > can consider disabling it for 5.2 too). The feature is upstream since > > Linux 5.8, I know that QEMU supports much older kernels but this doesn't > > probably mean that we can't enable new KVM PV features unless all > > supported kernels have it, we'd have to wait many years otherwise. > > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, > though that will go away in 6.1. > > We should take the occasion of dropping RHEL7 to be clearer about which > kernels are supported. It would be nice to be able to define sets of KVM functonality that we can either start given machine types with, or provide a separate switch to limit kvm functionality back to some defined point. We do trip over the same things pretty regularly when accidentally turning on new features. Dave > Paolo >
On Thu, Apr 15, 2021 at 08:14:30PM +0100, Dr. David Alan Gilbert wrote: > * Paolo Bonzini (pbonzini@redhat.com) wrote: > > On 06/04/21 13:42, Vitaly Kuznetsov wrote: > > > older machine types are still available (I disable it for <= 5.1 but we > > > can consider disabling it for 5.2 too). The feature is upstream since > > > Linux 5.8, I know that QEMU supports much older kernels but this doesn't > > > probably mean that we can't enable new KVM PV features unless all > > > supported kernels have it, we'd have to wait many years otherwise. > > > > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, > > though that will go away in 6.1. > > > > We should take the occasion of dropping RHEL7 to be clearer about which > > kernels are supported. > > It would be nice to be able to define sets of KVM functonality that we > can either start given machine types with, or provide a separate switch > to limit kvm functionality back to some defined point. We do trip over > the same things pretty regularly when accidentally turning on new > features. The same idea can apply to the hyperv=on stuff Vitaly is working on. Maybe we should consider making a generic version of the s390x FeatGroup code, use it to define convenient sets of KVM and hyperv features.
Eduardo Habkost <ehabkost@redhat.com> writes: > On Thu, Apr 15, 2021 at 08:14:30PM +0100, Dr. David Alan Gilbert wrote: >> * Paolo Bonzini (pbonzini@redhat.com) wrote: >> > On 06/04/21 13:42, Vitaly Kuznetsov wrote: >> > > older machine types are still available (I disable it for <= 5.1 but we >> > > can consider disabling it for 5.2 too). The feature is upstream since >> > > Linux 5.8, I know that QEMU supports much older kernels but this doesn't >> > > probably mean that we can't enable new KVM PV features unless all >> > > supported kernels have it, we'd have to wait many years otherwise. >> > >> > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, >> > though that will go away in 6.1. >> > >> > We should take the occasion of dropping RHEL7 to be clearer about which >> > kernels are supported. >> >> It would be nice to be able to define sets of KVM functonality that we >> can either start given machine types with, or provide a separate switch >> to limit kvm functionality back to some defined point. We do trip over >> the same things pretty regularly when accidentally turning on new >> features. > > The same idea can apply to the hyperv=on stuff Vitaly is working > on. Maybe we should consider making a generic version of the > s390x FeatGroup code, use it to define convenient sets of KVM and > hyperv features. True, the more I look at PV features enablement, the more I think that we're missing something important in the logic. All machine types we have are generally suposed to work with the oldest supported kernel so we should wait many years before enabling some of the new PV features (KVM or Hyper-V) by default. This also links to our parallel discussion regarding migration policies. Currently, we can't enable PV features by default based on their availability on the host because of migration, the set may differ on the destination host. What if we introduce (and maybe even switch to it by default) something like -migratable opportunistic (stupid name, I know) which would allow to enable all features supported by the source host and then somehow checking that the destination host has them all. This would effectively mean that it is possible to migrate a VM to a same-or-newer software (both kernel an QEMU) but not the other way around. This may be a reasonable choice.
On Wed, Apr 21, 2021 at 10:38:06AM +0200, Vitaly Kuznetsov wrote: > Eduardo Habkost <ehabkost@redhat.com> writes: > > > On Thu, Apr 15, 2021 at 08:14:30PM +0100, Dr. David Alan Gilbert wrote: > >> * Paolo Bonzini (pbonzini@redhat.com) wrote: > >> > On 06/04/21 13:42, Vitaly Kuznetsov wrote: > >> > > older machine types are still available (I disable it for <= 5.1 but we > >> > > can consider disabling it for 5.2 too). The feature is upstream since > >> > > Linux 5.8, I know that QEMU supports much older kernels but this doesn't > >> > > probably mean that we can't enable new KVM PV features unless all > >> > > supported kernels have it, we'd have to wait many years otherwise. > >> > > >> > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, > >> > though that will go away in 6.1. > >> > > >> > We should take the occasion of dropping RHEL7 to be clearer about which > >> > kernels are supported. > >> > >> It would be nice to be able to define sets of KVM functonality that we > >> can either start given machine types with, or provide a separate switch > >> to limit kvm functionality back to some defined point. We do trip over > >> the same things pretty regularly when accidentally turning on new > >> features. > > > > The same idea can apply to the hyperv=on stuff Vitaly is working > > on. Maybe we should consider making a generic version of the > > s390x FeatGroup code, use it to define convenient sets of KVM and > > hyperv features. > > True, the more I look at PV features enablement, the more I think that > we're missing something important in the logic. All machine types we > have are generally suposed to work with the oldest supported kernel so > we should wait many years before enabling some of the new PV features > (KVM or Hyper-V) by default. > > This also links to our parallel discussion regarding migration > policies. Currently, we can't enable PV features by default based on > their availability on the host because of migration, the set may differ > on the destination host. What if we introduce (and maybe even switch to > it by default) something like > > -migratable opportunistic (stupid name, I know) > > which would allow to enable all features supported by the source host > and then somehow checking that the destination host has them all. This > would effectively mean that it is possible to migrate a VM to a > same-or-newer software (both kernel an QEMU) but not the other way > around. This may be a reasonable choice. I don't think this is usable in pratice. Any large cloud or data center mgmt app using QEMU relies on migration, so can't opportunistically use arbitrary new features. They can only use features in the oldest kernel their deployment cares about. This can be newer than the oldest that QEMU supports, but still older than the newest that exists. ie we have situation where: - QEMU upstream minimum host is version 7 - Latest possible host is version 45 - A particular deployment has a mixture of hosts at version 24 and 37 "-migratable opportunistic" would let QEMU use features from version 37 despite the deployment needing compatibility with host version 24 still. It is almost as if we need to have a way to explicitly express a minimum required host version that VM requires compatibility with, so deployments can set their own baseline that is newer than QEMU minimum. Regards, Daniel
* Daniel P. Berrangé (berrange@redhat.com) wrote: > On Wed, Apr 21, 2021 at 10:38:06AM +0200, Vitaly Kuznetsov wrote: > > Eduardo Habkost <ehabkost@redhat.com> writes: > > > > > On Thu, Apr 15, 2021 at 08:14:30PM +0100, Dr. David Alan Gilbert wrote: > > >> * Paolo Bonzini (pbonzini@redhat.com) wrote: > > >> > On 06/04/21 13:42, Vitaly Kuznetsov wrote: > > >> > > older machine types are still available (I disable it for <= 5.1 but we > > >> > > can consider disabling it for 5.2 too). The feature is upstream since > > >> > > Linux 5.8, I know that QEMU supports much older kernels but this doesn't > > >> > > probably mean that we can't enable new KVM PV features unless all > > >> > > supported kernels have it, we'd have to wait many years otherwise. > > >> > > > >> > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, > > >> > though that will go away in 6.1. > > >> > > > >> > We should take the occasion of dropping RHEL7 to be clearer about which > > >> > kernels are supported. > > >> > > >> It would be nice to be able to define sets of KVM functonality that we > > >> can either start given machine types with, or provide a separate switch > > >> to limit kvm functionality back to some defined point. We do trip over > > >> the same things pretty regularly when accidentally turning on new > > >> features. > > > > > > The same idea can apply to the hyperv=on stuff Vitaly is working > > > on. Maybe we should consider making a generic version of the > > > s390x FeatGroup code, use it to define convenient sets of KVM and > > > hyperv features. > > > > True, the more I look at PV features enablement, the more I think that > > we're missing something important in the logic. All machine types we > > have are generally suposed to work with the oldest supported kernel so > > we should wait many years before enabling some of the new PV features > > (KVM or Hyper-V) by default. > > > > This also links to our parallel discussion regarding migration > > policies. Currently, we can't enable PV features by default based on > > their availability on the host because of migration, the set may differ > > on the destination host. What if we introduce (and maybe even switch to > > it by default) something like > > > > -migratable opportunistic (stupid name, I know) > > > > which would allow to enable all features supported by the source host > > and then somehow checking that the destination host has them all. This > > would effectively mean that it is possible to migrate a VM to a > > same-or-newer software (both kernel an QEMU) but not the other way > > around. This may be a reasonable choice. > > I don't think this is usable in pratice. Any large cloud or data center > mgmt app using QEMU relies on migration, so can't opportunistically > use arbitrary new features. They can only use features in the oldest > kernel their deployment cares about. This can be newer than the oldest > that QEMU supports, but still older than the newest that exists. > > ie we have situation where: > > - QEMU upstream minimum host is version 7 > - Latest possible host is version 45 > - A particular deployment has a mixture of hosts at version 24 and 37 > > "-migratable opportunistic" would let QEMU use features from version 37 > despite the deployment needing compatibility with host version 24 still. > > > It is almost as if we need to have a way to explicitly express a minimum > required host version that VM requires compatibility with, so deployments > can set their own baseline that is newer than QEMU minimum. It's not a 'version' - it's just the set of capabilities, and the qemu needs to check them at startup and fail if they're missing; I think that's what thats FeatGroup is that was suggested. Just like we have machine type and CPU version we need a set of PV features that we rely on the host kernel having, and we should only expose those PV features to the guest. It's possible that we might define some machine types as relying on certain PV features, or that some PV features wouldn't make sense on some machine types. 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 :|
Daniel P. Berrangé <berrange@redhat.com> writes: > On Wed, Apr 21, 2021 at 10:38:06AM +0200, Vitaly Kuznetsov wrote: >> Eduardo Habkost <ehabkost@redhat.com> writes: >> >> > On Thu, Apr 15, 2021 at 08:14:30PM +0100, Dr. David Alan Gilbert wrote: >> >> * Paolo Bonzini (pbonzini@redhat.com) wrote: >> >> > On 06/04/21 13:42, Vitaly Kuznetsov wrote: >> >> > > older machine types are still available (I disable it for <= 5.1 but we >> >> > > can consider disabling it for 5.2 too). The feature is upstream since >> >> > > Linux 5.8, I know that QEMU supports much older kernels but this doesn't >> >> > > probably mean that we can't enable new KVM PV features unless all >> >> > > supported kernels have it, we'd have to wait many years otherwise. >> >> > >> >> > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, >> >> > though that will go away in 6.1. >> >> > >> >> > We should take the occasion of dropping RHEL7 to be clearer about which >> >> > kernels are supported. >> >> >> >> It would be nice to be able to define sets of KVM functonality that we >> >> can either start given machine types with, or provide a separate switch >> >> to limit kvm functionality back to some defined point. We do trip over >> >> the same things pretty regularly when accidentally turning on new >> >> features. >> > >> > The same idea can apply to the hyperv=on stuff Vitaly is working >> > on. Maybe we should consider making a generic version of the >> > s390x FeatGroup code, use it to define convenient sets of KVM and >> > hyperv features. >> >> True, the more I look at PV features enablement, the more I think that >> we're missing something important in the logic. All machine types we >> have are generally suposed to work with the oldest supported kernel so >> we should wait many years before enabling some of the new PV features >> (KVM or Hyper-V) by default. >> >> This also links to our parallel discussion regarding migration >> policies. Currently, we can't enable PV features by default based on >> their availability on the host because of migration, the set may differ >> on the destination host. What if we introduce (and maybe even switch to >> it by default) something like >> >> -migratable opportunistic (stupid name, I know) >> >> which would allow to enable all features supported by the source host >> and then somehow checking that the destination host has them all. This >> would effectively mean that it is possible to migrate a VM to a >> same-or-newer software (both kernel an QEMU) but not the other way >> around. This may be a reasonable choice. > > I don't think this is usable in pratice. Any large cloud or data center > mgmt app using QEMU relies on migration, so can't opportunistically > use arbitrary new features. They can only use features in the oldest > kernel their deployment cares about. This can be newer than the oldest > that QEMU supports, but still older than the newest that exists. > > ie we have situation where: > > - QEMU upstream minimum host is version 7 > - Latest possible host is version 45 > - A particular deployment has a mixture of hosts at version 24 and 37 > > "-migratable opportunistic" would let QEMU use features from version 37 > despite the deployment needing compatibility with host version 24 still. > True; I was not really thinking about 'big' clouds/data centers, these should have enough resources to carefully set all the required features and not rely on the 'default'. My thoughts were around using migration for host upgrade on smaller (several hosts) deployments and in this case it's probably fairly reasonable to require to start with the oldest host and upgrade them all if getting new features is one of the upgrade goals. > > It is almost as if we need to have a way to explicitly express a minimum > required host version that VM requires compatibility with, so deployments > can set their own baseline that is newer than QEMU minimum. Yes, maybe, but setting the baseline is also a non-trivial task: e.g. how would users know which PV features they can enable without going through Linux kernel logs or just trying them on the oldest kernel they need? This should probably be solved by some upper layer management app which would collect feature sets from all hosts and come up with a common subset. I'm not sure if this is done by some tools already.
* Vitaly Kuznetsov (vkuznets@redhat.com) wrote: > Daniel P. Berrangé <berrange@redhat.com> writes: > > > On Wed, Apr 21, 2021 at 10:38:06AM +0200, Vitaly Kuznetsov wrote: > >> Eduardo Habkost <ehabkost@redhat.com> writes: > >> > >> > On Thu, Apr 15, 2021 at 08:14:30PM +0100, Dr. David Alan Gilbert wrote: > >> >> * Paolo Bonzini (pbonzini@redhat.com) wrote: > >> >> > On 06/04/21 13:42, Vitaly Kuznetsov wrote: > >> >> > > older machine types are still available (I disable it for <= 5.1 but we > >> >> > > can consider disabling it for 5.2 too). The feature is upstream since > >> >> > > Linux 5.8, I know that QEMU supports much older kernels but this doesn't > >> >> > > probably mean that we can't enable new KVM PV features unless all > >> >> > > supported kernels have it, we'd have to wait many years otherwise. > >> >> > > >> >> > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, > >> >> > though that will go away in 6.1. > >> >> > > >> >> > We should take the occasion of dropping RHEL7 to be clearer about which > >> >> > kernels are supported. > >> >> > >> >> It would be nice to be able to define sets of KVM functonality that we > >> >> can either start given machine types with, or provide a separate switch > >> >> to limit kvm functionality back to some defined point. We do trip over > >> >> the same things pretty regularly when accidentally turning on new > >> >> features. > >> > > >> > The same idea can apply to the hyperv=on stuff Vitaly is working > >> > on. Maybe we should consider making a generic version of the > >> > s390x FeatGroup code, use it to define convenient sets of KVM and > >> > hyperv features. > >> > >> True, the more I look at PV features enablement, the more I think that > >> we're missing something important in the logic. All machine types we > >> have are generally suposed to work with the oldest supported kernel so > >> we should wait many years before enabling some of the new PV features > >> (KVM or Hyper-V) by default. > >> > >> This also links to our parallel discussion regarding migration > >> policies. Currently, we can't enable PV features by default based on > >> their availability on the host because of migration, the set may differ > >> on the destination host. What if we introduce (and maybe even switch to > >> it by default) something like > >> > >> -migratable opportunistic (stupid name, I know) > >> > >> which would allow to enable all features supported by the source host > >> and then somehow checking that the destination host has them all. This > >> would effectively mean that it is possible to migrate a VM to a > >> same-or-newer software (both kernel an QEMU) but not the other way > >> around. This may be a reasonable choice. > > > > I don't think this is usable in pratice. Any large cloud or data center > > mgmt app using QEMU relies on migration, so can't opportunistically > > use arbitrary new features. They can only use features in the oldest > > kernel their deployment cares about. This can be newer than the oldest > > that QEMU supports, but still older than the newest that exists. > > > > ie we have situation where: > > > > - QEMU upstream minimum host is version 7 > > - Latest possible host is version 45 > > - A particular deployment has a mixture of hosts at version 24 and 37 > > > > "-migratable opportunistic" would let QEMU use features from version 37 > > despite the deployment needing compatibility with host version 24 still. > > > > True; I was not really thinking about 'big' clouds/data centers, these > should have enough resources to carefully set all the required features > and not rely on the 'default'. My thoughts were around using migration > for host upgrade on smaller (several hosts) deployments and in this case > it's probably fairly reasonable to require to start with the oldest host > and upgrade them all if getting new features is one of the upgrade goals. It's not actually that simple. Small installations tend to have less spare hardware available and/or flexibility; if you've got say a 3 or 5 host cluster, once you start upgrading one node you've now got nowhere to go if you hit a problem. Dave > > > > It is almost as if we need to have a way to explicitly express a minimum > > required host version that VM requires compatibility with, so deployments > > can set their own baseline that is newer than QEMU minimum. > > Yes, maybe, but setting the baseline is also a non-trivial task: > e.g. how would users know which PV features they can enable without > going through Linux kernel logs or just trying them on the oldest kernel > they need? This should probably be solved by some upper layer management > app which would collect feature sets from all hosts and come up with a > common subset. I'm not sure if this is done by some tools already. > > -- > Vitaly >
On Wed, Apr 21, 2021 at 11:29:45AM +0200, Vitaly Kuznetsov wrote: > Daniel P. Berrangé <berrange@redhat.com> writes: > > > On Wed, Apr 21, 2021 at 10:38:06AM +0200, Vitaly Kuznetsov wrote: > >> Eduardo Habkost <ehabkost@redhat.com> writes: > >> > >> > On Thu, Apr 15, 2021 at 08:14:30PM +0100, Dr. David Alan Gilbert wrote: > >> >> * Paolo Bonzini (pbonzini@redhat.com) wrote: > >> >> > On 06/04/21 13:42, Vitaly Kuznetsov wrote: > >> >> > > older machine types are still available (I disable it for <= 5.1 but we > >> >> > > can consider disabling it for 5.2 too). The feature is upstream since > >> >> > > Linux 5.8, I know that QEMU supports much older kernels but this doesn't > >> >> > > probably mean that we can't enable new KVM PV features unless all > >> >> > > supported kernels have it, we'd have to wait many years otherwise. > >> >> > > >> >> > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, > >> >> > though that will go away in 6.1. > >> >> > > >> >> > We should take the occasion of dropping RHEL7 to be clearer about which > >> >> > kernels are supported. > >> >> > >> >> It would be nice to be able to define sets of KVM functonality that we > >> >> can either start given machine types with, or provide a separate switch > >> >> to limit kvm functionality back to some defined point. We do trip over > >> >> the same things pretty regularly when accidentally turning on new > >> >> features. > >> > > >> > The same idea can apply to the hyperv=on stuff Vitaly is working > >> > on. Maybe we should consider making a generic version of the > >> > s390x FeatGroup code, use it to define convenient sets of KVM and > >> > hyperv features. > >> > >> True, the more I look at PV features enablement, the more I think that > >> we're missing something important in the logic. All machine types we > >> have are generally suposed to work with the oldest supported kernel so > >> we should wait many years before enabling some of the new PV features > >> (KVM or Hyper-V) by default. > >> > >> This also links to our parallel discussion regarding migration > >> policies. Currently, we can't enable PV features by default based on > >> their availability on the host because of migration, the set may differ > >> on the destination host. What if we introduce (and maybe even switch to > >> it by default) something like > >> > >> -migratable opportunistic (stupid name, I know) > >> > >> which would allow to enable all features supported by the source host > >> and then somehow checking that the destination host has them all. This > >> would effectively mean that it is possible to migrate a VM to a > >> same-or-newer software (both kernel an QEMU) but not the other way > >> around. This may be a reasonable choice. > > > > I don't think this is usable in pratice. Any large cloud or data center > > mgmt app using QEMU relies on migration, so can't opportunistically > > use arbitrary new features. They can only use features in the oldest > > kernel their deployment cares about. This can be newer than the oldest > > that QEMU supports, but still older than the newest that exists. > > > > ie we have situation where: > > > > - QEMU upstream minimum host is version 7 > > - Latest possible host is version 45 > > - A particular deployment has a mixture of hosts at version 24 and 37 > > > > "-migratable opportunistic" would let QEMU use features from version 37 > > despite the deployment needing compatibility with host version 24 still. > > > > True; I was not really thinking about 'big' clouds/data centers, these > should have enough resources to carefully set all the required features > and not rely on the 'default'. My thoughts were around using migration > for host upgrade on smaller (several hosts) deployments and in this case > it's probably fairly reasonable to require to start with the oldest host > and upgrade them all if getting new features is one of the upgrade goals. > > It is almost as if we need to have a way to explicitly express a minimum > > required host version that VM requires compatibility with, so deployments > > can set their own baseline that is newer than QEMU minimum. > > Yes, maybe, but setting the baseline is also a non-trivial task: > e.g. how would users know which PV features they can enable without > going through Linux kernel logs or just trying them on the oldest kernel > they need? This should probably be solved by some upper layer management > app which would collect feature sets from all hosts and come up with a > common subset. I'm not sure if this is done by some tools already. I specifically didn't talk in terms of features, because the problem you describe is unreasonable to push onto applications. Rather QEMU could express host baseline - "host-v1" - features A and B - "host-v2" - features A, B and C - "host-v3" - features A, B, C, D, E and f The mgmt app / admin only has to know which QEMU host baselines their hosts support. Essentially this could be viewed as separating the host kernel dependant bits out of the machine type, into a separate configuration axis. Regards, Daniel
Daniel P. Berrangé <berrange@redhat.com> writes: > On Wed, Apr 21, 2021 at 11:29:45AM +0200, Vitaly Kuznetsov wrote: >> Daniel P. Berrangé <berrange@redhat.com> writes: >> >> > On Wed, Apr 21, 2021 at 10:38:06AM +0200, Vitaly Kuznetsov wrote: >> >> Eduardo Habkost <ehabkost@redhat.com> writes: >> >> >> >> > On Thu, Apr 15, 2021 at 08:14:30PM +0100, Dr. David Alan Gilbert wrote: >> >> >> * Paolo Bonzini (pbonzini@redhat.com) wrote: >> >> >> > On 06/04/21 13:42, Vitaly Kuznetsov wrote: >> >> >> > > older machine types are still available (I disable it for <= 5.1 but we >> >> >> > > can consider disabling it for 5.2 too). The feature is upstream since >> >> >> > > Linux 5.8, I know that QEMU supports much older kernels but this doesn't >> >> >> > > probably mean that we can't enable new KVM PV features unless all >> >> >> > > supported kernels have it, we'd have to wait many years otherwise. >> >> >> > >> >> >> > Yes, this is a known problem in fact. :( In 6.0 we even support RHEL 7, >> >> >> > though that will go away in 6.1. >> >> >> > >> >> >> > We should take the occasion of dropping RHEL7 to be clearer about which >> >> >> > kernels are supported. >> >> >> >> >> >> It would be nice to be able to define sets of KVM functonality that we >> >> >> can either start given machine types with, or provide a separate switch >> >> >> to limit kvm functionality back to some defined point. We do trip over >> >> >> the same things pretty regularly when accidentally turning on new >> >> >> features. >> >> > >> >> > The same idea can apply to the hyperv=on stuff Vitaly is working >> >> > on. Maybe we should consider making a generic version of the >> >> > s390x FeatGroup code, use it to define convenient sets of KVM and >> >> > hyperv features. >> >> >> >> True, the more I look at PV features enablement, the more I think that >> >> we're missing something important in the logic. All machine types we >> >> have are generally suposed to work with the oldest supported kernel so >> >> we should wait many years before enabling some of the new PV features >> >> (KVM or Hyper-V) by default. >> >> >> >> This also links to our parallel discussion regarding migration >> >> policies. Currently, we can't enable PV features by default based on >> >> their availability on the host because of migration, the set may differ >> >> on the destination host. What if we introduce (and maybe even switch to >> >> it by default) something like >> >> >> >> -migratable opportunistic (stupid name, I know) >> >> >> >> which would allow to enable all features supported by the source host >> >> and then somehow checking that the destination host has them all. This >> >> would effectively mean that it is possible to migrate a VM to a >> >> same-or-newer software (both kernel an QEMU) but not the other way >> >> around. This may be a reasonable choice. >> > >> > I don't think this is usable in pratice. Any large cloud or data center >> > mgmt app using QEMU relies on migration, so can't opportunistically >> > use arbitrary new features. They can only use features in the oldest >> > kernel their deployment cares about. This can be newer than the oldest >> > that QEMU supports, but still older than the newest that exists. >> > >> > ie we have situation where: >> > >> > - QEMU upstream minimum host is version 7 >> > - Latest possible host is version 45 >> > - A particular deployment has a mixture of hosts at version 24 and 37 >> > >> > "-migratable opportunistic" would let QEMU use features from version 37 >> > despite the deployment needing compatibility with host version 24 still. >> > >> >> True; I was not really thinking about 'big' clouds/data centers, these >> should have enough resources to carefully set all the required features >> and not rely on the 'default'. My thoughts were around using migration >> for host upgrade on smaller (several hosts) deployments and in this case >> it's probably fairly reasonable to require to start with the oldest host >> and upgrade them all if getting new features is one of the upgrade goals. > > >> > It is almost as if we need to have a way to explicitly express a minimum >> > required host version that VM requires compatibility with, so deployments >> > can set their own baseline that is newer than QEMU minimum. >> >> Yes, maybe, but setting the baseline is also a non-trivial task: >> e.g. how would users know which PV features they can enable without >> going through Linux kernel logs or just trying them on the oldest kernel >> they need? This should probably be solved by some upper layer management >> app which would collect feature sets from all hosts and come up with a >> common subset. I'm not sure if this is done by some tools already. > > I specifically didn't talk in terms of features, because the problem you > describe is unreasonable to push onto applications. > > Rather QEMU could express host baseline > > - "host-v1" - features A and B > - "host-v2" - features A, B and C > - "host-v3" - features A, B, C, D, E and f > > The mgmt app / admin only has to know which QEMU host baselines their > hosts support. > > Essentially this could be viewed as separating the host kernel dependant > bits out of the machine type, into a separate configuration axis. In case we only think about upstream kernels and assuming PV features never go away that coud work. Distro kernels, however, exist too and feature backports are common, so which version should I declare when my kernel has e.g. features A, B and E ? (There used to be KVM_GET_API_VERSION ioctl but then we switched to CAPs and this happened for a reason.) Personaly, I'd vote for having individual PV features in the config if it ever gets introduced.