[PATCH v5] KVM: emulate: Don't inject #GP when emulating RDMPC if CR0.PE=0

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

SDM mentioned that, RDPMC:

  IF (((CR4.PCE = 1) or (CPL = 0) or (CR0.PE = 0)) and (ECX indicates a supported counter))
      THEN
          EAX := counter[31:0];
          EDX := ZeroExtend(counter[MSCB:32]);
      ELSE (* ECX is not valid or CR4.PCE is 0 and CPL is 1, 2, or 3 and CR0.PE is 1 *)
          #GP(0);
  FI;

Let's add a comment why CR0.PE isn't tested since it's impossible for CPL to be >0 if 
CR0.PE=0.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
v4 -> v5:
 * just comments
v3 -> v4:
 * add comments instead of pseudocode
v2 -> v3:
 * add the missing 'S'
v1 -> v2:
 * update patch description

 arch/x86/kvm/emulate.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 9a144ca8e146..c289809beea3 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4222,6 +4222,9 @@ static int check_rdpmc(struct x86_emulate_ctxt *ctxt)
 	if (enable_vmware_backdoor && is_vmware_backdoor_pmc(rcx))
 		return X86EMUL_CONTINUE;
 
+	/*
+	 * It's impossible for CPL to be >0 if CR0.PE=0.
+	 */
 	if ((!(cr4 & X86_CR4_PCE) && ctxt->ops->cpl(ctxt)) ||
 	    ctxt->ops->check_pmc(ctxt, rcx))
 		return emulate_gp(ctxt, 0);
-- 
2.25.1