[PATCH 1/3] KVM: SVM: Disable x2AVIC RDMSR interception for MSRs KVM actually supports

Sean Christopherson
2026-04-09T22:24:54Z

Fix multiple (classes of) bugs with one stone by using KVM's mask of
readable local APIC registers to determine which x2APIC MSRs to pass
through (or not) when toggling x2AVIC on/off. The existing hand-coded
list of MSRs is wrong on multiple fronts:

- ARBPRI, DFR, and ICR2 aren't supported by x2APIC; disabling
interception is nonsensical and suboptimal (the access generates a
#VMEXIT that requires decoding the instruction).

- RRR is completely unsupported.

- AVIC currently fails to pass through the "range of vectors" registers,
IRR, ISR, and TMR, as e.g. X2APIC_MSR(APIC_IRR) only affects IRR0, and
thus only disables intercept for vectors 31:0 (which are the *least*
interesting registers).

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 | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index adf211860949..df974ee290d0 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -122,6 +122,9 @@ static u32 x2avic_max_physical_id;
static void avic_set_x2apic_msr_interception(struct vcpu_svm *svm,
bool intercept)
{
+ struct kvm_vcpu *vcpu = &svm->vcpu;
+ u64 x2apic_readable_mask;
+
static const u32 x2avic_passthrough_msrs[] = {
X2APIC_MSR(APIC_ID),
X2APIC_MSR(APIC_LVR),
@@ -162,9 +165,15 @@ static void avic_set_x2apic_msr_interception(struct vcpu_svm *svm,
if (!x2avic_enabled)
return;

+ x2apic_readable_mask = kvm_lapic_readable_reg_mask(vcpu->arch.apic);
+
+ for (i = 0; i < BITS_PER_TYPE(typeof(x2apic_readable_mask)); i++)
+ svm_set_intercept_for_msr(vcpu, APIC_BASE_MSR + i,
+ MSR_TYPE_R, intercept);
+
for (i = 0; i < ARRAY_SIZE(x2avic_passthrough_msrs); i++)
- svm_set_intercept_for_msr(&svm->vcpu, x2avic_passthrough_msrs[i],
- MSR_TYPE_RW, intercept);
+ svm_set_intercept_for_msr(vcpu, x2avic_passthrough_msrs[i],
+ MSR_TYPE_W, intercept);

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