linux-kernel - [PATCH 2/2] KVM: SVM: Set/clear CR8 write interception when AVIC is (de)activated

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Message-ID: <20260203190711.458413-3-seanjc@google.com>
Date: Tue,  3 Feb 2026 11:07:10 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Jim Mattson <jmattson@...gle.com>, Naveen N Rao <naveen@...nel.org>, 
	"Maciej S . Szmigiero" <maciej.szmigiero@...cle.com>
Subject: [PATCH 2/2] KVM: SVM: Set/clear CR8 write interception when AVIC is (de)activated

Explicitly set/clear CR8 write interception when AVIC is (de)activated to
fix a bug where KVM leaves the interception enabled after AVIC is
activated.  E.g. if KVM emulates INIT=>WFS while AVIC is deactivated, CR8
will remain intercepted in perpetuity.

On its own, the dangling CR8 intercept is "just" a performance issue, but
combined with the TPR sync bug fixed by commit d02e48830e3f ("KVM: SVM:
Sync TPR from LAPIC into VMCB::V_TPR even if AVIC is active"), the danging
intercept is fatal to Windows guests as the TPR seen by hardware gets
wildly out of sync with reality.

Note, VMX isn't affected by the bug as TPR_THRESHOLD is explicitly ignored
when Virtual Interrupt Delivery is enabled, i.e. when APICv is active in
KVM's world.  I.e. there's no need to trigger update_cr8_intercept(), this
is firmly an SVM implementation flaw/detail.

WARN if KVM gets a CR8 write #VMEXIT while AVIC is active, as KVM should
never enter the guest with AVIC enabled and CR8 writes intercepted.

Fixes: 3bbf3565f48c ("svm: Do not intercept CR8 when enable AVIC")
Cc: stable@...r.kernel.org
Cc: Jim Mattson <jmattson@...gle.com>
Cc: Naveen N Rao (AMD) <naveen@...nel.org>
Cc: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/avic.c | 6 ++++--
 arch/x86/kvm/svm/svm.c  | 9 +++++----
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 44e07c27b190..13a4a8949aba 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -189,12 +189,12 @@ static void avic_activate_vmcb(struct vcpu_svm *svm)
 	struct kvm_vcpu *vcpu = &svm->vcpu;
 
 	vmcb->control.int_ctl &= ~(AVIC_ENABLE_MASK | X2APIC_MODE_MASK);
-
 	vmcb->control.avic_physical_id &= ~AVIC_PHYSICAL_MAX_INDEX_MASK;
 	vmcb->control.avic_physical_id |= avic_get_max_physical_id(vcpu);
-
 	vmcb->control.int_ctl |= AVIC_ENABLE_MASK;
 
+	svm_clr_intercept(svm, INTERCEPT_CR8_WRITE);
+
 	/*
 	 * Note: KVM supports hybrid-AVIC mode, where KVM emulates x2APIC MSR
 	 * accesses, while interrupt injection to a running vCPU can be
@@ -226,6 +226,8 @@ static void avic_deactivate_vmcb(struct vcpu_svm *svm)
 	vmcb->control.int_ctl &= ~(AVIC_ENABLE_MASK | X2APIC_MODE_MASK);
 	vmcb->control.avic_physical_id &= ~AVIC_PHYSICAL_MAX_INDEX_MASK;
 
+	svm_set_intercept(svm, INTERCEPT_CR8_WRITE);
+
 	/*
 	 * If running nested and the guest uses its own MSR bitmap, there
 	 * is no need to update L0's msr bitmap
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e8313fdc5465..aa3ab22215f5 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1077,8 +1077,7 @@ static void init_vmcb(struct kvm_vcpu *vcpu, bool init_event)
 	svm_set_intercept(svm, INTERCEPT_CR0_WRITE);
 	svm_set_intercept(svm, INTERCEPT_CR3_WRITE);
 	svm_set_intercept(svm, INTERCEPT_CR4_WRITE);
-	if (!kvm_vcpu_apicv_active(vcpu))
-		svm_set_intercept(svm, INTERCEPT_CR8_WRITE);
+	svm_set_intercept(svm, INTERCEPT_CR8_WRITE);
 
 	set_dr_intercepts(svm);
 
@@ -2674,9 +2673,11 @@ static int dr_interception(struct kvm_vcpu *vcpu)
 
 static int cr8_write_interception(struct kvm_vcpu *vcpu)
 {
-	int r;
-
 	u8 cr8_prev = kvm_get_cr8(vcpu);
+	int r;
+
+	WARN_ON_ONCE(kvm_vcpu_apicv_active(vcpu));
+
 	/* instruction emulation calls kvm_set_cr8() */
 	r = cr_interception(vcpu);
 	if (lapic_in_kernel(vcpu))
-- 
2.53.0.rc2.204.g2597b5adb4-goog