linux-kernel - [PATCH 04/10] KVM: x86: nSVM: Restore L1's PAT on emulated #VMEXIT from L2 to L1

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Message-ID: <20260113003016.3511895-5-jmattson@google.com>
Date: Mon, 12 Jan 2026 16:29:59 -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>, Alexander Graf <agraf@...e.de>, 
	"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
Cc: Jim Mattson <jmattson@...gle.com>
Subject: [PATCH 04/10] KVM: x86: nSVM: Restore L1's PAT on emulated #VMEXIT
 from L2 to L1

KVM doesn't implement a separate G_PAT register to hold the guest's
PAT in guest mode with nested NPT enabled. Consequently, L1's IA32_PAT
MSR must be restored on emulated #VMEXIT from L2 to L1.

Note: if L2 uses shadow paging, L1 and L2 share the same IA32_PAT MSR.

Signed-off-by: Jim Mattson <jmattson@...gle.com>
---
 arch/x86/kvm/svm/nested.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index c751be470364..9aec836ac04c 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1292,6 +1292,16 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
 	kvm_rsp_write(vcpu, vmcb01->save.rsp);
 	kvm_rip_write(vcpu, vmcb01->save.rip);
 
+	/*
+	 * KVM doesn't implement a separate guest PAT
+	 * register. Instead, the guest PAT lives in vcpu->arch.pat
+	 * while in guest mode with nested NPT enabled. Hence, the
+	 * IA32_PAT MSR has to be restored from the vmcb01 g_pat at
+	 * #VMEXIT.
+	 */
+	if (nested_npt_enabled(svm))
+		vcpu->arch.pat = vmcb01->save.g_pat;
+
 	svm->vcpu.arch.dr7 = DR7_FIXED_1;
 	kvm_update_dr7(&svm->vcpu);
 
-- 
2.52.0.457.g6b5491de43-goog