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

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Message-ID: <87a5gntzd5.fsf@redhat.com>
Date: Wed, 04 Sep 2024 13:47:02 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Yan Zhao <yan.y.zhao@...el.com>, Gerd Hoffmann <kraxel@...hat.com>,
 Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
 rcu@...r.kernel.org, linux-kernel@...r.kernel.org, Kevin Tian
 <kevin.tian@...el.com>, Yiwei Zhang <zzyiwei@...gle.com>, Lai Jiangshan
 <jiangshanlai@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, Josh
 Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 5/5] KVM: VMX: Always honor guest PAT on CPUs that
 support self-snoop

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> Sean Christopherson <seanjc@...gle.com> writes:
>
>> On Mon, Sep 02, 2024, Vitaly Kuznetsov wrote:
>>> FWIW, I use QEMU-9.0 from the same C10S (qemu-kvm-9.0.0-7.el10.x86_64)
>>> but I don't think it matters in this case. My CPU is "Intel(R) Xeon(R)
>>> Silver 4410Y".
>>
>> Has this been reproduced on any other hardware besides SPR?  I.e. did we stumble
>> on another hardware issue?
>
> Very possible, as according to Yan Zhao this doesn't reproduce on at
> least "Coffee Lake-S". Let me try to grab some random hardware around
> and I'll be back with my observations.

We have some interesting results :-)

In addition to Sapphire Rapids, the issue also reproduces on a Cascade
lake CPU (Intel(R) Xeon(R) Silver 4214 CPU) but does NOT reproduce on
Skylake (Intel(R) Xeon(R) Gold 5118 CPU). I don't have a lot of desktop
CPUs around, so can't say much.

AMD also doesn't seem to be affected, at leats AMD EPYC 7413 works fine.

-- 
Vitaly