linux-kernel - [PATCH 2/2] KVM: x86: Advertise support for WRMSRNS

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Message-ID: <20250227010111.3222742-3-seanjc@google.com>
Date: Wed, 26 Feb 2025 17:01:11 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, Xin Li <xin@...or.com>
Subject: [PATCH 2/2] KVM: x86: Advertise support for WRMSRNS

Advertise support for WRMSRNS (WRMSR non-serializing) to userspace if the
instruction is supported by the underlying CPU.  From a virtualization
perspective, the only difference between WRMSRNS and WRMSR is that VM-Exits
due to WRMSRNS set EXIT_QUALIFICATION to '1'.  WRMSRNS doesn't require a
new enabling control, shares the same basic exit reason, and behaves the
same as WRMSR with respect to MSR interception.

  WRMSR and WRMSRNS use the same basic exit reason (see Appendix C). For
  WRMSR, the exit qualification is 0, while for WRMSRNS it is 1.

Don't do anything different when emulating WRMSRNS vs. WRMSR, as KVM can't
do anything less, i.e. can't make emulation non-serializing.  The
motivation for the guest to use WRMSRNS instead of WRMSR is to avoid
immediately serializing the CPU when the necessary serialization is
guaranteed by some other mechanism, i.e. WRMSRNS being fully serializing
isn't guest-visible, just less performant.

Suggested-by: Xin Li (Intel) <xin@...or.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/cpuid.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 97a90689a9dc..ebecfe4bea1e 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -992,6 +992,7 @@ void kvm_set_cpu_caps(void)
 		F(FZRM),
 		F(FSRS),
 		F(FSRC),
+		F(WRMSRNS),
 		F(AMX_FP16),
 		F(AVX_IFMA),
 		F(LAM),
-- 
2.48.1.711.g2feabab25a-goog