[PATCH v3 16/20] KVM: x86: Guard valid XCR0.APX settings

Chang S. Bae
2026-04-28T05:26:37Z

Prevent invalid XCR0.APX configurations in two cases: conflict with MPX
and lack of SVM support.

In the non-compacted XSAVE format, APX and MPX conflict on the same
offset. Although MPX is being deprecated in practice, KVM should
explicitly reject such configurations that set both bits.

At this point, only VMX supports EGPRs. SVM will require corresponding
extensions to handle EGPR indices.

Updating the supported XCR0 mask will be done separately with CPUID
exposure.

Link: https://lore.kernel.org/ab3f4937-38f5-4354-8850-bf773c159bbe@redhat.com
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
---
arch/x86/kvm/svm/svm.c | 7 ++++++-
arch/x86/kvm/x86.c | 4 ++++
2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 73febbb24340..080bc2f829d9 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5546,8 +5546,13 @@ static __init int svm_hardware_setup(void)
return -EOPNOTSUPP;
}

+ /*
+ * APX introduces EGPRs, which require additional VMCB support.
+ * Disable APX until the necessary extensions are handled.
+ */
kvm_caps.supported_xcr0 &= ~(XFEATURE_MASK_BNDREGS |
- XFEATURE_MASK_BNDCSR);
+ XFEATURE_MASK_BNDCSR |
+ XFEATURE_MASK_APX);

if (tsc_scaling) {
if (!boot_cpu_has(X86_FEATURE_TSCRATEMSR)) {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fb77869f0233..b6b2fd877893 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1290,6 +1290,10 @@ int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
(!(xcr0 & XFEATURE_MASK_BNDCSR)))
return 1;

+ /* MPX and APX conflict in the non-compacted XSAVE format */
+ if (xcr0 & XFEATURE_MASK_BNDREGS && xcr0 & XFEATURE_MASK_APX)
+ return 1;
+
if (xcr0 & XFEATURE_MASK_AVX512) {
if (!(xcr0 & XFEATURE_MASK_YMM))
return 1;
--
2.51.0