KVM: emulate: Comment on difference between RDPMC implementation and manual

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.
Signed-off-by: NWanpeng Li <wanpengli@tencent.com>
Message-Id: <1634724836-73721-1-git-send-email-wanpengli@tencent.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -4223,6 +4223,11 @@ static int check_rdpmc(struct x86_emulate_ctxt *ctxt)
 	if (enable_vmware_backdoor && is_vmware_backdoor_pmc(rcx))
 		return X86EMUL_CONTINUE;
 
+	/*
+	 * If CR4.PCE is set, the SDM requires CPL=0 or CR0.PE=0.  The CR0.PE
+	 * check however is unnecessary because CPL is always 0 outside
+	 * protected mode.
+	 */
 	if ((!(cr4 & X86_CR4_PCE) && ctxt->ops->cpl(ctxt)) ||
 	    ctxt->ops->check_pmc(ctxt, rcx))
 		return emulate_gp(ctxt, 0);