KVM: x86 emulator: introduce NoWrite flag

Instead of disabling writeback via OP_NONE, just specify NoWrite.
Acked-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi.kivity@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/emulate.c

@@ -151,6 +151,7 @@
 #define Unaligned   ((u64)1 << 42)  /* Explicitly unaligned (e.g. MOVDQU) */
 #define Avx         ((u64)1 << 43)  /* Advanced Vector Extensions */
 #define Fastop      ((u64)1 << 44)  /* Use opcode::u.fastop */
+#define NoWrite     ((u64)1 << 45)  /* No writeback */
 
 #define X2(x...) x, x
 #define X3(x...) X2(x), x
@@ -1633,6 +1634,9 @@ static int writeback(struct x86_emulate_ctxt *ctxt)
 {
 	int rc;
 
+	if (ctxt->d & NoWrite)
+		return X86EMUL_CONTINUE;
+
 	switch (ctxt->dst.type) {
 	case OP_REG:
 		write_register_operand(&ctxt->dst);