[PATCH 5/5] KVM: x86: Expose Zhaoxin RSA CPUID feature

From: Ewan Hai

Date: Wed May 13 2026 - 04:33:17 EST

Advertise the Zhaoxin big-number arithmetic engine to guests viaCPUID 0xC0000001 EDX bits 27 (RSA) and 28 (RSA_EN). The RSA featureprovides two user-mode instructions for modular arithmetic on bigintegers, documented in the Zhaoxin PadLock Instruction Reference,chapter 4 ("Modular Multiplication and Exponentiation Engine"). Bothsupport operand sizes from 256 to 32768 bits (in 128-bit increments): - REP XMODEXP (encoding F3 0F A6 F8, subsection 4.1) computes A^B mod M - REP MONTMUL2 (encoding F3 0F A6 F0, subsection 4.2) computes A*B mod MREP MONTMUL2 is the long-mode replacement of legacy REP MONTMUL, whichis restricted to compatibility and 32-bit protected modes. Theseprimitives accelerate RSA and related public-key operations.Both instructions are user-mode and available in all CPU modes, with noassociated MSR control. The RSA and RSA_EN bits are redundant byhardware design (set or cleared together) and both serve purely asCPUID-level feature-presence reporting flags requiring no KVMemulation. Both bits are advertised because different software mayprobe either one when checking for RSA availability.Signed-off-by: Ewan Hai <ewanhai-oc@xxxxxxxxxxx>--- arch/x86/kvm/cpuid.c | 2 ++ arch/x86/kvm/reverse_cpuid.h | 2 ++ 2 files changed, 4 insertions(+)diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.cindex 3fb81f7a6107..94ea9abae566 100644--- a/arch/x86/kvm/cpuid.c+++ b/arch/x86/kvm/cpuid.c@@ -1290,6 +1290,8 @@ void kvm_initialize_cpu_caps(void) F(RNG2_EN), F(PHE2), F(PHE2_EN),+ F(RSA),+ F(RSA_EN), ); /*diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.hindex f28300c2d5e0..0df96ff9515c 100644--- a/arch/x86/kvm/reverse_cpuid.h+++ b/arch/x86/kvm/reverse_cpuid.h@@ -85,6 +85,8 @@ #define X86_FEATURE_RNG2_EN KVM_X86_FEATURE(CPUID_C000_0001_EDX, 23) #define X86_FEATURE_PHE2 KVM_X86_FEATURE(CPUID_C000_0001_EDX, 25) #define X86_FEATURE_PHE2_EN KVM_X86_FEATURE(CPUID_C000_0001_EDX, 26)+#define X86_FEATURE_RSA KVM_X86_FEATURE(CPUID_C000_0001_EDX, 27)+#define X86_FEATURE_RSA_EN KVM_X86_FEATURE(CPUID_C000_0001_EDX, 28) struct cpuid_reg { u32 function;-- 2.34.1

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