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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Aug 06, 2025 at 07:57:34PM +0200, Christian Ehrhardt wrote:
> 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?

No, that commit message is misleading.

IIUC mark_unavailable_features() actively blocks usage of the feature,
so it is a functional change, not merely a emitting warning.

It makes me wonder if that commit was actually intended to block the
feature or not, vs merely warning ?  CC'ing those involved in the
commit.

With regards,
Daniel
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