KVM: vmx: do not unconditionally clear EFER switching

Do not unconditionally call clear_atomic_switch_msr() when updating
EFER.  This adds up to four unnecessary VMWrites in the case where
guest_efer != host_efer, e.g. if the load_on_{entry,exit} bits were
already set.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -2790,8 +2790,6 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
 		ignore_bits &= ~(u64)EFER_SCE;
 #endif
 
-	clear_atomic_switch_msr(vmx, MSR_EFER);
-
 	/*
 	 * On EPT, we can't emulate NX, so we must switch EFER atomically.
 	 * On CPUs that support "load IA32_EFER", always switch EFER
@@ -2804,8 +2802,12 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
 		if (guest_efer != host_efer)
 			add_atomic_switch_msr(vmx, MSR_EFER,
 					      guest_efer, host_efer, false);
+		else
+			clear_atomic_switch_msr(vmx, MSR_EFER);
 		return false;
 	} else {
+		clear_atomic_switch_msr(vmx, MSR_EFER);
+
 		guest_efer &= ~ignore_bits;
 		guest_efer |= host_efer & ignore_bits;