[PATCH 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 user-mode 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/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.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),
 	);
 
 	/*
diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
index 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