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

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Message-ID: <df3c6560-dd37-4ec2-9b7e-1ad4c3ceba07@leemhuis.info>
Date: Mon, 7 Oct 2024 15:28:38 +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 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?

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