linux-kernel - Re: [PATCH] KVM: VMX: Add a quirk to (not) honor guest PAT on CPUs that support self-snoop

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Message-ID: <Z+yOQiB77bts6dO9@yzhao56-desk.sh.intel.com>
Date: Wed, 2 Apr 2025 09:09:22 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: VMX: Add a quirk to (not) honor guest PAT on CPUs
 that support self-snoop

On Tue, Apr 01, 2025 at 05:22:38PM -0700, Sean Christopherson wrote:
> On Wed, Apr 02, 2025, Yan Zhao wrote:
> > On Tue, Apr 01, 2025 at 03:11:07PM -0700, Sean Christopherson wrote:
> > > Add back support for honoring guest PAT on Intel CPUs that support self-
> > > snoop (and don't have errata), but guarded by a quirk so as not to break
> > > existing setups that subtly relied on KVM forcing WB for synthetic
> > > devices.
> > > 
> > > This effectively reverts commit 9d70f3fec14421e793ffbc0ec2f739b24e534900
> > > and reapplies 377b2f359d1f71c75f8cc352b5c81f2210312d83, but with a quirk.
> > > 
> > > Cc: Yan Zhao <yan.y.zhao@...el.com>
> > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > ---
> > >
> > Hi Sean,
> > 
> > > AFAIK, we don't have an answer as to whether the slow UC behavior on CLX+
> > > is working as intended or a CPU flaw, which Paolo was hoping we would get
> > We did answer the slow UC behavior is working as intended at [1].
> > 
> > "After consulting with CPU architects,
> >  it's told that this behavior is expected on ICX/SPR Xeon platforms due to
> >  the snooping implementation."
> > 
> > Paolo then help update the series to v2 [2] /v3 [3].
> > 
> > Did you overlook those series, or is there something I missed?
> 
> Nope, you didn't miss anything.  I have that series in my TODO folder, but only
> glanced at it when it flew by and completely missed that it quirks ignoring
> guest PAT.  Not sure how I missed the cover letter subject though...
> 
> Anyways, ignore this, my bad.  Thanks for the update, and sorry for the noise!
That's OK :)