Caching qemu capabilities and KubeVirt

Roman Mohr rmohr at google.com
Tue Sep 13 15:31:42 UTC 2022 

On Thu, Sep 8, 2022 at 4:32 PM Daniel P. Berrangé <berrange at redhat.com>
wrote:

> On Thu, Sep 08, 2022 at 04:22:17PM +0200, Roman Mohr wrote:
> > On Thu, Sep 8, 2022 at 4:04 PM Daniel P. Berrangé <berrange at redhat.com>
> > wrote:
> >
> > > On Thu, Sep 08, 2022 at 02:23:31PM +0100, Daniel P. Berrangé wrote:
> > > > On Thu, Sep 08, 2022 at 03:10:09PM +0200, Roman Mohr wrote:
> > > > > On Thu, Sep 8, 2022 at 2:56 PM Daniel P. Berrangé <
> berrange at redhat.com
> > > >
> > > > > wrote:
> > > > >
> > > > > > On Thu, Sep 08, 2022 at 02:24:00PM +0200, Roman Mohr wrote:
> > > > > > > Hi,
> > > > > > >
> > > > > > >
> > > > > > > I have a question regarding capability caching  in the context
> of
> > > > > > KubeVirt.
> > > > > > > Since we start in KubeVirt one libvirt instance per VM, libvirt
> > > has to
> > > > > > > re-discover on every VM start the qemu capabilities which
> leads to
> > > a
> > > > > > 1-2s+
> > > > > > > delay in startup.
> > > > > > >
> > > > > > > We already discover the features in a dedicated KubeVirt pod on
> > > each
> > > > > > node.
> > > > > > > Therefore I tried to copy the capabilities over to see if that
> > > would
> > > > > > work.
> > > > > > >
> > > > > > > It looks like in general it could work, but libvirt seems to
> > > detect a
> > > > > > > mismatch in the exposed KVM CPU ID in every pod. Therefore it
> > > invalidates
> > > > > > > the cache. The recreated capability cache looks esctly like the
> > > original
> > > > > > > one though ...
> > > > > > >
> > > > > > > The check responsible for the invalidation is this:
> > > > > > >
> > > > > > > ```
> > > > > > > Outdated capabilities for '%s': host cpuid changed
> > > > > > > ```
> > > > > > >
> > > > > > > So the KVM_GET_SUPPORTED_CPUID call seems to return
> > > > > > > slightly different values in different containers.
> > > > > > >
> > > > > > > After trying out the attached golang scripts in different
> > > containers, I
> > > > > > > could indeed see differences.
> > > > > > >
> > > > > > > I can however not really judge what the differences in these
> KVM
> > > function
> > > > > > > registers mean and I am curious if someone else knows. The
> files
> > > are
> > > > > > > attached too (as json for easy diffing).
> > > > > >
> > > > > > Can you confirm whether the two attached data files were captured
> > > > > > by containers running on the same physical host, or could each
> > > > > > container have run on a different host.
> > > > > >
> > > > >
> > > > > They are coming from the same host, that is the most surprising bit
> > > for me.
> > > > > I am also very sure that this is the case, because I only had one
> k8s
> > > node
> > > > > from where I took these.
> > > > > The containers however differ (obviously) on namespaces and on the
> > > > > privilege level (less obvious). The handler dump is from a fully
> > > privileged
> > > > > container.
> > > >
> > > > The privilege level sounds like something that might be impactful,
> > > > so I'll investigate that.  I'd be pretty surprised for namespaces
> > > > to have any impact thnough.
> > >
> > > The privilege level is a red herring. Peter reminded me that we have
> > > to filter out some parts of CPUID because the APIC IDs vary depending
> > > on what host CPU the task executes on.
> > >
> > >
> > >
> https://gitlab.com/libvirt/libvirt/-/blob/master/src/util/virhostcpu.c#L1346
> > >
> > > In the 2 jSON files you provide, the differences i see should already
> > > be matched by
> > >
> > >         /* filter out local apic id */
> > >         if (entry->function == 0x01 && entry->index == 0x00)
> > >             entry->ebx &= 0x00ffffff;
> > >         if (entry->function == 0x0b)
> > >             entry->edx &= 0xffffff00;
> > >
> > > so those differences ought not to be causing the cache to be
> > > invalidated.
> > >
> >
> > Hm, maybe I misinterpreted the logs then. The snipped I looked at was
> this:
> >
> >
> > ```
> > {"component":"virt-launcher","level":"info","msg":"/dev/kvm has changed
> > (1661786802 vs
> >
> 0)","pos":"virQEMUCapsKVMUsable:4850","subcomponent":"libvirt","thread":"25","timestamp":"2022-08-29T15:26:42.936000Z"}
> > {"component":"virt-launcher","level":"info","msg":"a=0x7f8138153ba0,
> >
> b=0x7f818001c480","pos":"virCPUDataIsIdentical:1178","subcomponent":"libvirt","thread":"25","timestamp":"2022-08-29T15:26:42.939000Z"}
> > {"component":"virt-launcher","level":"info","msg":"Outdated capabilities
> > for '/usr/bin/qemu-system-x86_64': host cpuid
> >
> changed","pos":"virQEMUCapsIsValid:4993","subcomponent":"libvirt","thread":"25","timestamp":"2022-08-29T15:26:42.939000Z"}
> > {"component":"virt-launcher","level":"info","msg":"Outdated cached
> > capabilities
> >
> '/var/cache/libvirt/qemu/capabilities/926803a9278e445ec919c2b6cbd8c1c449c75b26dcb1686b774314180376c725.xml'
> > for
> >
> '/usr/bin/qemu-system-x86_64'","pos":"virFileCacheLoad:163","subcomponent":"libvirt","thread":"25","timestamp":"2022-08-29T15:26:42.939000Z"}
> > ```
>
> Can you capture the
>
>
> /var/cache/libvirt/qemu/capabilities/926803a9278e445ec919c2b6cbd8c1c449c75b26dcb1686b774314180376c725.xml'


> from the virt-handler and virt-launcher pods. It contains a <cpuid>
> block that will show us the differences libvirt recored, /after/,
> libvirt has done its filtering. This will show if there si more
> we need to filter.
>

Done. Only attached the one from handler since it is 100% identical to the
launcher.

Let me know if you need more information.

Thanks and best regards,
Roman


>
> > I had the impression from the code that the `/dev/kvm` change (because
> the
> > containers are not created at the same time) does not invalidate it
> either.
> >
> > I added the whole debug log, maybe I missed something obvious.
> >
> > Does it make a difference if the cache is created via `virsh
> > domcapabilities` and `virsh capabilities` or via defining the first
> domain?
>
> They'll all end up at the same caching scode so should not make any
> difference.
>
> >
> > Best regards,
> > Roman
> >
> >
> > >
> > > With 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 :|
> > >
> > >
>
>
>
> With 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 :|
>
>
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