KVM: x86 emulator: Disable writeback for CMP emulation

This stops "CMP r/m, reg" to write back the data into memory.
Pointed out by Avi.

The writeback suppression now covers CMP, CMPS, SCAS.
Signed-off-by: NTakuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: NAvi Kivity <avi@redhat.com>

arch/x86/kvm/emulate.c

@@ -3671,6 +3671,7 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 		break;
 	case 0x38 ... 0x3d:
 	      cmp:		/* cmp */
+		c->dst.type = OP_NONE; /* Disable writeback. */
 		emulate_2op_SrcV("cmp", c->src, c->dst, ctxt->eflags);
 		break;
 	case 0x40 ... 0x47: /* inc r16/r32 */
@@ -3797,7 +3798,6 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 		rc = emulate_popf(ctxt, ops, &c->dst.val, c->op_bytes);
 		break;
 	case 0xa6 ... 0xa7:	/* cmps */
-		c->dst.type = OP_NONE; /* Disable writeback. */
 		goto cmp;
 	case 0xa8 ... 0xa9:	/* test ax, imm */
 		goto test;