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

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Message-ID: <Zt+bTJvNgkG4JeD8@yzhao56-desk.sh.intel.com>
Date: Tue, 10 Sep 2024 09:05:16 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>
CC: Sean Christopherson <seanjc@...gle.com>, Vitaly Kuznetsov
	<vkuznets@...hat.com>, Gerd Hoffmann <kraxel@...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

On Mon, Sep 09, 2024 at 03:24:40PM +0200, Paolo Bonzini wrote:
> While this is a fix for future kernels, it doesn't change the result for VMs
> already in existence.
Though this is the truth, I have concerns that there may be other guest drivers
with improper PAT configurations that were previously masked by KVM's force-WB
setting. Now that we respect the guest's PAT settings, these misconfigurations
could lead to degraded performance, potentially perceived as errors, as was
observed in the previous VMX unit test and the current Bochs scenario.

> I don't think there's an alternative to putting this behind a quirk.