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

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Message-ID: <87iku4ghiw.fsf@redhat.com>
Date: Mon, 07 Oct 2024 15:38:15 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: "Linux regression tracking (Thorsten Leemhuis)"
 <regressions@...mhuis.info>, 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

"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"

Also, there's a (pending) DRM patch fixing it from the guest's side:
https://gitlab.freedesktop.org/drm/misc/kernel/-/commit/9388ccf69925223223c87355a417ba39b13a5e8e

-- 
Vitaly