linux-kernel - [PATCH 03/10] KVM: x86: nSVM: Handle legacy SVM nested state in SET_NESTED

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Message-ID: <20260113003016.3511895-4-jmattson@google.com>
Date: Mon, 12 Jan 2026 16:29:58 -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 03/10] KVM: x86: nSVM: Handle legacy SVM nested state in SET_NESTED_STATE

Previously, KVM didn't record the vmcb01 G_PAT (i.e. the IA32_PAT MSR)
in the serialized nested state. It didn't have to, because it ignored
the vmcb12 g_pat field entirely. L1 and L2 simply shared the same PAT.

To preserve legacy behavior, copy the current value of the IA32_PAT
MSR to the location of the vmcb01 G_PAT in the serialized nested
state.  (On restore, KVM_SET_MSRS should be called before
KVM_SET_NESTED_STATE, so the value of the shared IA32_PAT MSR should
be available.)

Signed-off-by: Jim Mattson <jmattson@...gle.com>
---
 arch/x86/kvm/svm/nested.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index ed24e08d2d21..c751be470364 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1884,6 +1884,13 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
         if (((cr0 & X86_CR0_CD) == 0) && (cr0 & X86_CR0_NW))
 		goto out_free;
 
+	/*
+	 * If kvm_state doesn't have a valid saved L1 g_pat, use the
+	 * PAT MSR instead. This preserves the legacy behavior.
+	 */
+	if (!(kvm_state->hdr.svm.flags & KVM_STATE_SVM_VALID_GPAT))
+		save->g_pat = vcpu->arch.pat;
+
 	/*
 	 * Validate host state saved from before VMRUN (see
 	 * nested_svm_check_permissions).
-- 
2.52.0.457.g6b5491de43-goog