[PATCH RFC v1 19/20] KVM: x86: Expose APX sub-features to guests

Chang S. Bae
2025-11-10T18:25:10Z

Add CPUID leaf 0x29 sub-leaf 0 to enumerate APX sub-features to guests.
This leaf currently defines the following sub-features:

* New Conditional Instructions (NCI)
* New Data Destination (NDD)
* Flags Suppression (NF)

The CPUID leaf is only exposed if the APX feature is enabled.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
---
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/cpuid.c | 10 ++++++++--
arch/x86/kvm/reverse_cpuid.h | 4 ++++
3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 940f83c121cf..763872080c64 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -794,6 +794,7 @@ enum kvm_only_cpuid_leafs {
CPUID_24_0_EBX,
CPUID_8000_0021_ECX,
CPUID_7_1_ECX,
+ CPUID_29_0_EBX,
NR_KVM_CPU_CAPS,

NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index b90e58f2a42f..95c25de641ca 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1063,6 +1063,10 @@ void kvm_set_cpu_caps(void)
F(AVX10_512),
);

+ kvm_cpu_cap_init(CPUID_29_0_EBX,
+ F(APX_NCI_NDD_NF),
+ );
+
kvm_cpu_cap_init(CPUID_8000_0001_ECX,
F(LAHF_LM),
F(CMP_LEGACY),
@@ -1640,8 +1644,10 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
break;
}
case 0x29: {
- /* No APX sub-features are supported yet */
- entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
+ if (!(kvm_caps.supported_xcr0 & XFEATURE_MASK_APX)) {
+ entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
+ break;
+ }
break;
}
case KVM_CPUID_SIGNATURE: {
diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
index e9d9fb4070ca..a8eca23ee2d4 100644
--- a/arch/x86/kvm/reverse_cpuid.h
+++ b/arch/x86/kvm/reverse_cpuid.h
@@ -50,6 +50,9 @@
#define X86_FEATURE_AVX10_256 KVM_X86_FEATURE(CPUID_24_0_EBX, 17)
#define X86_FEATURE_AVX10_512 KVM_X86_FEATURE(CPUID_24_0_EBX, 18)

+/* Intel-defined sub-features, CPUID level 0x00000029:0 (EBX) */
+#define X86_FEATURE_APX_NCI_NDD_NF KVM_X86_FEATURE(CPUID_29_0_EBX, 0)
+
/* CPUID level 0x80000007 (EDX). */
#define KVM_X86_FEATURE_CONSTANT_TSC KVM_X86_FEATURE(CPUID_8000_0007_EDX, 8)

@@ -92,6 +95,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
[CPUID_24_0_EBX] = { 0x24, 0, CPUID_EBX},
[CPUID_8000_0021_ECX] = {0x80000021, 0, CPUID_ECX},
[CPUID_7_1_ECX] = { 7, 1, CPUID_ECX},
+ [CPUID_29_0_EBX] = { 0x29, 0, CPUID_EBX},
};

/*
--
2.51.0