Re: Issues with pdcm in qemu 10.1-rc on migration and save/restore

[Christian Ehrhardt <christian.ehrhardt@canonical.com>]

On Wed, Aug 6, 2025 at 2:00 PM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Wed, Aug 06, 2025 at 01:52:17PM +0200, Christian Ehrhardt wrote:
> > Hi,
> > I was unsure if this would be better sent to libvirt or qemu - the
> > issue is somewhere between libvirt modelling CPUs and qemu 10.1
> > behaving differently. I did not want to double post and gladly most of
> > the people are on both lists - since the switch in/out of the problem
> > is qemu 10.0 <-> 10.1 let me start here. I beg your pardon for not yet
> > having all the answers, I'm sure I could find more with debugging, but
> > I also wanted to report early for your awareness while we are still in
> > the RC phase.
> >
> >
> > # Problem
> >
> > What I found when testing migrations in Ubuntu with qemu 10.1-rc1 was:
> >   error: operation failed: guest CPU doesn't match specification:
> > missing features: pdcm
> >
> > This is behaving the same with libvirt 11.4 or the more recent 11.6.
> > But switching back to qemu 10.0 confirmed that this behavior is new
> > with qemu 10.1-rc.
>
>
> > Without yet having any hard evidence against them I found a few pdcm
> > related commits between 10.0 and 10.1-rc1:
> >   7ff24fb65 i386/tdx: Don't mask off CPUID_EXT_PDCM
> >   00268e000 i386/cpu: Warn about why CPUID_EXT_PDCM is not available
> >   e68ec2980 i386/cpu: Move adjustment of CPUID_EXT_PDCM before
> > feature_dependencies[] check
> >   0ba06e46d i386/tdx: Add TDX fixed1 bits to supported CPUIDs
> >
> >
> > # Caveat
> >
> > My test environment is in LXD system containers, that gives me issues
> > in the power management detection
> >   libvirtd[406]: error from service: GDBus.Error:System.Error.EROFS:
> > Read-only file system
> >   libvirtd[406]: Failed to get host power management capabilities
>
> That's harmless.

Yeah, it always was for me - thanks for confirming.

> > And the resulting host-model on a  rather old test server will therefore have:
> >   <cpu mode='custom' match='exact' check='full'>
> >     <model fallback='forbid'>Haswell-noTSX-IBRS</model>
> >     <vendor>Intel</vendor>
> >     <feature policy='require' name='vmx'/>
> >     <feature policy='disable' name='pdcm'/>
> >      ...
> >
> > But that was fine in the past, and the behavior started to break
> > save/restore or migrations just now with the new qemu 10.1-rc.
> >
> > # Next steps
> >
> > I'm soon overwhelmed by meetings for the rest of the day, but would be
> > curious if one has a suggestion about what to look at next for
> > debugging or a theory about what might go wrong. If nothing else comes
> > up I'll try to set up a bisect run tomorrow.
>
> Yeah, git bisect is what I'd start with.

Bisect complete, identified this commit

commit 00268e00027459abede448662f8794d78eb4b0a4
Author: Xiaoyao Li <xiaoyao.li@intel.com>
Date:   Tue Mar 4 00:24:50 2025 -0500

    i386/cpu: Warn about why CPUID_EXT_PDCM is not available

    When user requests PDCM explicitly via "+pdcm" without PMU enabled, emit
    a warning to inform the user.

    Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
    Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
    Link: https://lore.kernel.org/r/20250304052450.465445-3-xiaoyao.li@intel.com
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

 target/i386/cpu.c | 3 +++
 1 file changed, 3 insertions(+)



Which is odd as it should only add a warning right?
But I checked the logs - the build on "e68ec29809 i386/cpu: Move
adjustment of CPUID_EXT_PDCM before feature_dependencies[] check"
passed the same use case.
I'll build both outside of the bisect run tomorrow to ensure this is
reproducible when I watch it more closely (than submitting a bisect
script).
Maybe this already helps to put your eyes and thoughts in the right direction.

> With regards,
> Daniel
> --
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>


-- 
Christian Ehrhardt
Director of Engineering, Ubuntu Server
Canonical Ltd