[PATCH 3/3] KVM: SVM: Only disable x2AVIC WRMSR interception for MSRs that are accelerated

Sean Christopherson
2026-04-09T22:24:58Z

When x2AVIC is enabled, disable WRMSR interception only for MSRs that are
actually accelerated by hardware. Disabling interception for MSRs that
aren't accelerated is functionally "fine", but very suboptimal as many
accesses generate AVIC_UNACCELERATED_ACCESS fault #VMEXITs, which requires
KVM to decode the instruction to figure out what the guest was trying to
access.

Note, the set of MSRs that are passed through for write is identical to
VMX's set when IPI virtualization is enabled. This is not a coincidence,
as x2AVIC is functionally equivalent to APICv+IPIv.

Fixes: 4d1d7942e36a ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")
Cc: stable@vger.kernel.org
Cc: Naveen N Rao (AMD) <naveen@kernel.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/avic.c | 40 ++++------------------------------------
1 file changed, 4 insertions(+), 36 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index c9e9872ad880..2b07cc347b90 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -124,39 +124,6 @@ static void avic_set_x2apic_msr_interception(struct vcpu_svm *svm,
{
struct kvm_vcpu *vcpu = &svm->vcpu;
u64 x2apic_readable_mask;
-
- static const u32 x2avic_passthrough_msrs[] = {
- X2APIC_MSR(APIC_ID),
- X2APIC_MSR(APIC_LVR),
- X2APIC_MSR(APIC_TASKPRI),
- X2APIC_MSR(APIC_ARBPRI),
- X2APIC_MSR(APIC_PROCPRI),
- X2APIC_MSR(APIC_EOI),
- X2APIC_MSR(APIC_RRR),
- X2APIC_MSR(APIC_LDR),
- X2APIC_MSR(APIC_DFR),
- X2APIC_MSR(APIC_SPIV),
- X2APIC_MSR(APIC_ISR),
- X2APIC_MSR(APIC_TMR),
- X2APIC_MSR(APIC_IRR),
- X2APIC_MSR(APIC_ESR),
- X2APIC_MSR(APIC_ICR),
- X2APIC_MSR(APIC_ICR2),
-
- /*
- * Note! Always intercept LVTT, as TSC-deadline timer mode
- * isn't virtualized by hardware, and the CPU will generate a
- * #GP instead of a #VMEXIT.
- */
- X2APIC_MSR(APIC_LVTTHMR),
- X2APIC_MSR(APIC_LVTPC),
- X2APIC_MSR(APIC_LVT0),
- X2APIC_MSR(APIC_LVT1),
- X2APIC_MSR(APIC_LVTERR),
- X2APIC_MSR(APIC_TMICT),
- X2APIC_MSR(APIC_TMCCT),
- X2APIC_MSR(APIC_TDCR),
- };
int i;

if (intercept == svm->x2avic_msrs_intercepted)
@@ -174,9 +141,10 @@ static void avic_set_x2apic_msr_interception(struct vcpu_svm *svm,
if (!intercept)
svm_enable_intercept_for_msr(vcpu, X2APIC_MSR(APIC_TMCCT), MSR_TYPE_R);

- for (i = 0; i < ARRAY_SIZE(x2avic_passthrough_msrs); i++)
- svm_set_intercept_for_msr(vcpu, x2avic_passthrough_msrs[i],
- MSR_TYPE_W, intercept);
+ svm_set_intercept_for_msr(vcpu, X2APIC_MSR(APIC_TASKPRI), MSR_TYPE_W, intercept);
+ svm_set_intercept_for_msr(vcpu, X2APIC_MSR(APIC_EOI), MSR_TYPE_W, intercept);
+ svm_set_intercept_for_msr(vcpu, X2APIC_MSR(APIC_SELF_IPI), MSR_TYPE_W, intercept);
+ svm_set_intercept_for_msr(vcpu, X2APIC_MSR(APIC_ICR), MSR_TYPE_W, intercept);

svm->x2avic_msrs_intercepted = intercept;
}
--
2.53.0.1213.gd9a14994de-goog