linux-kernel - [PATCH v2 14/16] KVM: x86: Expose APX foundational feature bit to guests

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Message-ID: <20260112235408.168200-15-chang.seok.bae@intel.com>
Date: Mon, 12 Jan 2026 23:54:06 +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,
	Peter Fang <peter.fang@...el.com>
Subject: [PATCH v2 14/16] KVM: x86: Expose APX foundational feature bit to guests

From: Peter Fang <peter.fang@...el.com>

Add the APX xfeature bit to the list of supported XCR0 components and
expose the APX feature to guests. Update the maximum supported CPUID leaf
to 0x29 to include the APX leaf. On SVM systems, ensure that the feature
is not advertised as EGPR support is not yet supported.

No APX sub-features are enumerated yet. Those will be exposed in a
separate patch.

Signed-off-by: Peter Fang <peter.fang@...el.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
---
V1 -> V2: Exclude the APX CPUID definition as patch8 includes it now
---
 arch/x86/kvm/cpuid.c   | 8 +++++++-
 arch/x86/kvm/svm/svm.c | 8 ++++++++
 arch/x86/kvm/x86.c     | 3 ++-
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 88a5426674a1..5431e31a4851 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1038,6 +1038,7 @@ void kvm_set_cpu_caps(void)
 		F(AVX_VNNI_INT16),
 		F(PREFETCHITI),
 		F(AVX10),
+		SCATTERED_F(APX),
 	);
 
 	kvm_cpu_cap_init(CPUID_7_2_EDX,
@@ -1401,7 +1402,7 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 	switch (function) {
 	case 0:
 		/* Limited to the highest leaf implemented in KVM. */
-		entry->eax = min(entry->eax, 0x24U);
+		entry->eax = min(entry->eax, 0x29U);
 		break;
 	case 1:
 		cpuid_entry_override(entry, CPUID_1_EDX);
@@ -1646,6 +1647,11 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		entry->edx = 0;
 		break;
 	}
+	case 0x29: {
+		/* No APX sub-features are supported yet */
+		entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
+		break;
+	}
 	case KVM_CPUID_SIGNATURE: {
 		const u32 *sigptr = (const u32 *)KVM_SIGNATURE;
 		entry->eax = KVM_CPUID_FEATURES;
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index a06e5a24b808..9c76ea7a4231 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5284,6 +5284,14 @@ static __init void svm_set_cpu_caps(void)
 	 */
 	kvm_cpu_cap_clear(X86_FEATURE_BUS_LOCK_DETECT);
 	kvm_cpu_cap_clear(X86_FEATURE_MSR_IMM);
+
+	/*
+	 * If the APX xfeature bit is not supported, meaning that VMCB
+	 * support for EGPRs is unavailable, then the APX feature should
+	 * not be exposed to the guest.
+	 */
+	if (!(kvm_caps.supported_xcr0 & XFEATURE_MASK_APX))
+		kvm_cpu_cap_clear(X86_FEATURE_APX);
 }
 
 static __init int svm_hardware_setup(void)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 189a03483d03..67b3312ab737 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -214,7 +214,8 @@ static DEFINE_PER_CPU(struct kvm_user_return_msrs, user_return_msrs);
 #define KVM_SUPPORTED_XCR0     (XFEATURE_MASK_FP | XFEATURE_MASK_SSE \
 				| XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \
 				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
-				| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
+				| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE \
+				| XFEATURE_MASK_APX)
 
 #define XFEATURE_MASK_CET_ALL	(XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL)
 /*
-- 
2.51.0