x86, kvm, vmx: Don't set LOAD_IA32_EFER when host and guest match

There's nothing to switch if the host and guest values are the same.
I am unable to find evidence that this makes any difference
whatsoever.
Signed-off-by: NAndy Lutomirski <luto@amacapital.net>
[I could see a difference on Nehalem.  From 5 runs:

 userspace exit, guest!=host   12200 11772 12130 12164 12327
 userspace exit, guest=host    11983 11780 11920 11919 12040
 lightweight exit, guest!=host  3214  3220  3238  3218  3337
 lightweight exit, guest=host   3178  3193  3193  3187  3220

 This passes the t-test with 99% confidence for userspace exit,
 98.5% confidence for lightweight exit. - Paolo]
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -1673,7 +1673,9 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
 		guest_efer = vmx->vcpu.arch.efer;
 		if (!(guest_efer & EFER_LMA))
 			guest_efer &= ~EFER_LME;
-		add_atomic_switch_msr(vmx, MSR_EFER, guest_efer, host_efer);
+		if (guest_efer != host_efer)
+			add_atomic_switch_msr(vmx, MSR_EFER,
+					      guest_efer, host_efer);
 		return false;
 	}