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 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.

> 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.

With regards,
Daniel
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