On 07.10.24 15:38, Vitaly Kuznetsov wrote: > "Linux regression tracking (Thorsten Leemhuis)" > <regressions@xxxxxxxxxxxxx> writes: > >> On 30.08.24 11:35, Vitaly Kuznetsov wrote: >>> Sean Christopherson <seanjc@xxxxxxxxxx> writes: >>> >>>> Unconditionally honor guest PAT on CPUs that support self-snoop, as >>>> Intel has confirmed that CPUs that support self-snoop always snoop caches >>>> and store buffers. I.e. CPUs with self-snoop maintain cache coherency >>>> even in the presence of aliased memtypes, thus there is no need to trust >>>> the guest behaves and only honor PAT as a last resort, as KVM does today. >>>> >>>> Honoring guest PAT is desirable for use cases where the guest has access >>>> to non-coherent DMA _without_ bouncing through VFIO, e.g. when a virtual >>>> (mediated, for all intents and purposes) GPU is exposed to the guest, along >>>> with buffers that are consumed directly by the physical GPU, i.e. which >>>> can't be proxied by the host to ensure writes from the guest are performed >>>> with the correct memory type for the GPU. >>> >>> Necroposting! >>> >>> Turns out that this change broke "bochs-display" driver in QEMU even >>> when the guest is modern (don't ask me 'who the hell uses bochs for >>> modern guests', it was basically a configuration error :-). E.g: >>> [...] >> >> This regression made it to the list of tracked regressions. It seems >> this thread stalled a while ago. Was this ever fixed? Does not look like >> it, but I might have missed something. Or is this a regression I should >> just ignore for one reason or another? >> > > The regression was addressed in by reverting 377b2f359d1f in 6.11 > > commit 9d70f3fec14421e793ffbc0ec2f739b24e534900 > Author: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Date: Sun Sep 15 02:49:33 2024 -0400 > > Revert "KVM: VMX: Always honor guest PAT on CPUs that support self-snoop" Thx. Sorry, missed that, thx for pointing me towards it. I had looked for things like that, but seems I messed up my lore query. Apologies for the noise! > Also, there's a (pending) DRM patch fixing it from the guest's side: > https://gitlab.freedesktop.org/drm/misc/kernel/-/commit/9388ccf69925223223c87355a417ba39b13a5e8e Great! Ciao, Thorsten P.S.: #regzbot fix: 9d70f3fec14421e793ffbc0ec2f739b24e534900
