[PATCH v2 5/5] KVM: x86: Expose Zhaoxin RSA CPUID feature

[Ewan Hai <ewandevelop@gmail.com>]

Advertise the Zhaoxin big-number arithmetic engine to guests via
CPUID 0xC0000001 EDX bits 27 (RSA) and 28 (RSA_EN). The RSA feature
provides two user-mode instructions for modular arithmetic on big
integers, documented in the Zhaoxin PadLock Instruction Reference,
chapter 4 ("Modular Multiplication and Exponentiation Engine"). Both
support 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 M

REP MONTMUL2 is the long-mode replacement of legacy REP MONTMUL, which
is restricted to compatibility and 32-bit protected modes. These
primitives accelerate RSA and related public-key operations.

Both instructions are unprivileged (no CPL restriction) and available
in all CPU modes, with no associated MSR control. The RSA and RSA_EN
bits are redundant by hardware design (set or cleared together) and
both serve purely as CPUID-level feature-presence reporting flags
requiring no KVM emulation. Both bits are advertised because different
software may probe either one when checking for RSA availability.

Signed-off-by: Ewan Hai <ewandevelop@gmail.com>
---
 arch/x86/include/asm/cpufeatures.h | 2 ++
 arch/x86/kvm/cpuid.c               | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 3702d7a30ae6..a769c83588f7 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -154,6 +154,8 @@
 #define X86_FEATURE_RNG2_EN		( 5*32+23) /* "rng2_en" RNG2 enabled */
 #define X86_FEATURE_PHE2		( 5*32+25) /* "phe2" PadLock Hash Engine v2 */
 #define X86_FEATURE_PHE2_EN		( 5*32+26) /* "phe2_en" PHE2 enabled */
+#define X86_FEATURE_RSA			( 5*32+27) /* "rsa" Big-number arithmetic */
+#define X86_FEATURE_RSA_EN		( 5*32+28) /* "rsa_en" RSA enabled */
 
 /* More extended AMD flags: CPUID level 0x80000001, ECX, word 6 */
 #define X86_FEATURE_LAHF_LM		( 6*32+ 0) /* "lahf_lm" LAHF/SAHF in long mode */
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 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),
 	);
 
 	/*
-- 
2.34.1