linux-kernel - [PATCH v2 1/2] KVM: SVM: Sync TPR from LAPIC into VMCB::V

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Message-ID: <c231be64280b1461e854e1ce3595d70cde3a2e9d.1756139678.git.maciej.szmigiero@oracle.com>
Date: Mon, 25 Aug 2025 18:44:28 +0200
From: "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To: Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>
Cc: Maxim Levitsky <mlevitsk@...hat.com>,
	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
	Naveen N Rao <naveen@...nel.org>,
	Alejandro Jimenez <alejandro.j.jimenez@...cle.com>,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/2] KVM: SVM: Sync TPR from LAPIC into VMCB::V_TPR even if AVIC is active

From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>

Commit 3bbf3565f48c ("svm: Do not intercept CR8 when enable AVIC")
inhibited pre-VMRUN sync of TPR from LAPIC into VMCB::V_TPR in
sync_lapic_to_cr8() when AVIC is active.

AVIC does automatically sync between these two fields, however it does
so only on explicit guest writes to one of these fields, not on a bare
VMRUN.

This meant that when AVIC is enabled host changes to TPR in the LAPIC
state might not get automatically copied into the V_TPR field of VMCB.

This is especially true when it is the userspace setting LAPIC state via
KVM_SET_LAPIC ioctl() since userspace does not have access to the guest
VMCB.

Practice shows that it is the V_TPR that is actually used by the AVIC to
decide whether to issue pending interrupts to the CPU (not TPR in TASKPRI),
so any leftover value in V_TPR will cause serious interrupt delivery issues
in the guest when AVIC is enabled.

Fix this issue by doing pre-VMRUN TPR sync from LAPIC into VMCB::V_TPR
even when AVIC is enabled.

Fixes: 3bbf3565f48c ("svm: Do not intercept CR8 when enable AVIC")
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
---
 arch/x86/kvm/svm/svm.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index d9931c6c4bc6..1bfebe40854f 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4046,8 +4046,7 @@ static inline void sync_lapic_to_cr8(struct kvm_vcpu *vcpu)
 	struct vcpu_svm *svm = to_svm(vcpu);
 	u64 cr8;

-	if (nested_svm_virtualize_tpr(vcpu) ||
-	    kvm_vcpu_apicv_active(vcpu))
+	if (nested_svm_virtualize_tpr(vcpu))
 		return;

 	cr8 = kvm_get_cr8(vcpu);