Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that support self-snoop

[Linux regression tracking (Thorsten Leemhuis)]

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?


Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
Everything you wanna know about Linux kernel regression tracking:
https://linux-regtracking.leemhuis.info/about/#tldr
If I did something stupid, please tell me, as explained on that page.

#regzbot poke