KVM: x86 emulator: add decoding of CMPXCHG8B dst operand

Decode CMPXCHG8B destination operand in decoding stage. Fixes regression
introduced by "If LOCK prefix is used dest arg should be memory" commit.
This commit relies on dst operand be decoded at the beginning of an
instruction emulation.
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/emulate.c

@@ -52,6 +52,7 @@
 #define DstMem      (3<<1)	/* Memory operand. */
 #define DstAcc      (4<<1)      /* Destination Accumulator */
 #define DstDI       (5<<1)	/* Destination is in ES:(E)DI */
+#define DstMem64    (6<<1)	/* 64bit memory operand */
 #define DstMask     (7<<1)
 /* Source operand type. */
 #define SrcNone     (0<<4)	/* No source operand. */
@@ -360,7 +361,7 @@ static u32 group_table[] = {
 	DstMem | SrcImmByte | ModRM, DstMem | SrcImmByte | ModRM | Lock,
 	DstMem | SrcImmByte | ModRM | Lock, DstMem | SrcImmByte | ModRM | Lock,
 	[Group9*8] =
-	0, ImplicitOps | ModRM | Lock, 0, 0, 0, 0, 0, 0,
+	0, DstMem64 | ModRM | Lock, 0, 0, 0, 0, 0, 0,
 };
 
 static u32 group2_table[] = {
@@ -1205,6 +1206,7 @@ x86_decode_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 			 c->twobyte && (c->b == 0xb6 || c->b == 0xb7));
 		break;
 	case DstMem:
+	case DstMem64:
 		if ((c->d & ModRM) && c->modrm_mod == 3) {
 			c->dst.bytes = (c->d & ByteOp) ? 1 : c->op_bytes;
 			c->dst.type = OP_REG;
@@ -1214,6 +1216,9 @@ x86_decode_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 		}
 		c->dst.type = OP_MEM;
 		c->dst.ptr = (unsigned long *)c->modrm_ea;
+		if ((c->d & DstMask) == DstMem64)
+			c->dst.bytes = 8;
+		else
 			c->dst.bytes = (c->d & ByteOp) ? 1 : c->op_bytes;
 		c->dst.val = 0;
 		if (c->d & BitOp) {
@@ -1706,12 +1711,7 @@ static inline int emulate_grp9(struct x86_emulate_ctxt *ctxt,
 			       struct x86_emulate_ops *ops)
 {
 	struct decode_cache *c = &ctxt->decode;
-	u64 old, new;
-	int rc;
-
-	rc = ops->read_emulated(c->modrm_ea, &old, 8, ctxt->vcpu);
-	if (rc != X86EMUL_CONTINUE)
-		return rc;
+	u64 old = c->dst.orig_val;
 
 	if (((u32) (old >> 0) != (u32) c->regs[VCPU_REGS_RAX]) ||
 	    ((u32) (old >> 32) != (u32) c->regs[VCPU_REGS_RDX])) {
@@ -1719,15 +1719,12 @@ static inline int emulate_grp9(struct x86_emulate_ctxt *ctxt,
 		c->regs[VCPU_REGS_RAX] = (u32) (old >> 0);
 		c->regs[VCPU_REGS_RDX] = (u32) (old >> 32);
 		ctxt->eflags &= ~EFLG_ZF;
-
 	} else {
-		new = ((u64)c->regs[VCPU_REGS_RCX] << 32) |
+		c->dst.val = ((u64)c->regs[VCPU_REGS_RCX] << 32) |
 		       (u32) c->regs[VCPU_REGS_RBX];
 
-		rc = ops->cmpxchg_emulated(c->modrm_ea, &old, &new, 8, ctxt->vcpu);
-		if (rc != X86EMUL_CONTINUE)
-			return rc;
 		ctxt->eflags |= EFLG_ZF;
+		c->lock_prefix = 1;
 	}
 	return X86EMUL_CONTINUE;
 }
@@ -3245,7 +3242,6 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 		rc = emulate_grp9(ctxt, ops);
 		if (rc != X86EMUL_CONTINUE)
 			goto done;
-		c->dst.type = OP_NONE;
 		break;
 	}
 	goto writeback;