KVM: x86: Expose Zhaoxin RSA CPUID feature

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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 unprivileged 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.

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Ewan Hai <ewanhai-oc@zhaoxin.com>
Link: https://patch.msgid.link/20260610023512.3690734-6-ewanhai-oc@zhaoxin.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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Ewan Hai-oc 2026-06-10 10:35:12 +08:00 • committed by Sean Christopherson

parent 54db9aaacc

commit dcfb0f2c06

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arch/x86/include/asm/cpufeatures.h

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arch/x86/kvm/cpuid.c

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