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 -- |: 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 :|
