linux-kernel - [PATCH v2 13/16] KVM: x86: Guard valid XCR0.APX settings

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Message-ID: <20260112235408.168200-14-chang.seok.bae@intel.com>
Date: Mon, 12 Jan 2026 23:54:05 +0000
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: pbonzini@...hat.com,
	seanjc@...gle.com
Cc: kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	chao.gao@...el.com,
	chang.seok.bae@...el.com
Subject: [PATCH v2 13/16] KVM: x86: Guard valid XCR0.APX settings

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.

The addition to the supported XCR0 mask should accompany guest CPUID
exposure, which will be done separately.

Link: https://lore.kernel.org/ab3f4937-38f5-4354-8850-bf773c159bbe@redhat.com
Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@...el.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 209faa742e98..a06e5a24b808 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5301,8 +5301,13 @@ static __init int svm_hardware_setup(void)
 	}
 	kvm_enable_efer_bits(EFER_NX);
 
+	/*
+	 * 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 (boot_cpu_has(X86_FEATURE_FXSR_OPT))
 		kvm_enable_efer_bits(EFER_FFXSR);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e7f858488f2c..189a03483d03 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1391,6 +1391,10 @@ 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