linux-kernel - Re: [PATCH 00/10] KVM: x86: nSVM: Improve PAT virtualization

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Message-ID: <CALMp9eTLS6_HhWTkf-baga77=0zNq6KUBYtvY0WNuGs3ts2Ptw@mail.gmail.com>
Date: Tue, 13 Jan 2026 16:31:50 -0800
From: Jim Mattson <jmattson@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>, Joerg Roedel <joro@...tes.org>, 
	Avi Kivity <avi@...hat.com>, Radim Krčmář <rkrcmar@...hat.com>, 
	David Hildenbrand <david@...nel.org>, Cathy Avery <cavery@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 00/10] KVM: x86: nSVM: Improve PAT virtualization

On Mon, Jan 12, 2026 at 4:30 PM Jim Mattson <jmattson@...gle.com> wrote:
>
> KVM's implementation of nested SVM treats PAT the same way whether or
> not nested NPT is enabled: L1 and L2 share a PAT.
>
> This is correct when nested NPT is disabled, but incorrect when nested
> NPT is enabled. When nested NPT is enabled, L1 and L2 have independent
> PATs.

Yosry points out that this series does not correctly handle saving a
checkpoint on a new kernel and restoring it on an old kernel. In that
scenario, KVM_SET_MSRS will restore the L2 PAT, and the old kernel
will not restore L1's PAT on emulated #VMEXIT.

I have also discovered that not all userspace VMMs restore MSRs before
nested state.

Ironically, I think the way to correctly deal with compatibility in
both directions is to go back to the architected separation of hPAT
and gPAT. Accesses to IA32_PAT from userspace will always have to
reference hPAT to properly restore a new checkpoint on an old kernel.

Cooking up v2...