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

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Message-ID: <ba91a7cc-c647-4e33-82f3-0c4e52ce89ea@leemhuis.info>
Date: Mon, 7 Oct 2024 16:04:24 +0200
From: "Linux regression tracking (Thorsten Leemhuis)"
 <regressions@...mhuis.info>
To: Vitaly Kuznetsov <vkuznets@...hat.com>,
 Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
 Kevin Tian <kevin.tian@...el.com>, Yan Zhao <yan.y.zhao@...el.com>,
 Yiwei Zhang <zzyiwei@...gle.com>, Lai Jiangshan <jiangshanlai@...il.com>,
 "Paul E. McKenney" <paulmck@...nel.org>,
 Josh Triplett <josh@...htriplett.org>, Gerd Hoffmann <kraxel@...hat.com>,
 Linux kernel regressions list <regressions@...ts.linux.dev>
Subject: Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that support
 self-snoop

On 07.10.24 15:38, Vitaly Kuznetsov wrote:
> "Linux regression tracking (Thorsten Leemhuis)"
> <regressions@...mhuis.info> writes:
> 
>> On 30.08.24 11:35, Vitaly Kuznetsov wrote:
>>> Sean Christopherson <seanjc@...gle.com> 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@...hat.com>
> 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