KVM: x86 emulator: decode extended accumulator explicity

Single-operand MUL and DIV access an extended accumulator: AX for byte
instructions, and DX:AX, EDX:EAX, or RDX:RAX for larger-sized instructions.
Add support for fetching the extended accumulator.

In order not to change things too much, RDX is loaded into Src2, which is
already loaded by fastop().  This avoids increasing register pressure on
i386.

Gleb: disable src writeback for ByteOp div/mul.

Signed-off-by: Avi Kivity <avi.kivity@gmail.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

arch/x86/kvm/emulate.c

@@ -61,6 +61,8 @@
 #define OpMem8            26ull  /* 8-bit zero extended memory operand */
 #define OpImm64           27ull  /* Sign extended 16/32/64-bit immediate */
 #define OpXLat            28ull  /* memory at BX/EBX/RBX + zero-extended AL */
+#define OpAccLo           29ull  /* Low part of extended acc (AX/AX/EAX/RAX) */
+#define OpAccHi           30ull  /* High part of extended acc (-/DX/EDX/RDX) */
 
 #define OpBits             5  /* Width of operand field */
 #define OpMask             ((1ull << OpBits) - 1)
@@ -86,6 +88,7 @@
 #define DstMem64    (OpMem64 << DstShift)
 #define DstImmUByte (OpImmUByte << DstShift)
 #define DstDX       (OpDX << DstShift)
+#define DstAccLo    (OpAccLo << DstShift)
 #define DstMask     (OpMask << DstShift)
 /* Source operand type. */
 #define SrcShift    6
@@ -108,6 +111,7 @@
 #define SrcImm64    (OpImm64 << SrcShift)
 #define SrcDX       (OpDX << SrcShift)
 #define SrcMem8     (OpMem8 << SrcShift)
+#define SrcAccHi    (OpAccHi << SrcShift)
 #define SrcMask     (OpMask << SrcShift)
 #define BitOp       (1<<11)
 #define MemAbs      (1<<12)      /* Memory operand is absolute displacement */
@@ -157,6 +161,8 @@
 #define NoWrite     ((u64)1 << 45)  /* No writeback */
 #define SrcWrite    ((u64)1 << 46)  /* Write back src operand */
 
+#define DstXacc     (DstAccLo | SrcAccHi | SrcWrite)
+
 #define X2(x...) x, x
 #define X3(x...) X2(x), x
 #define X4(x...) X2(x), X2(x)
@@ -4166,6 +4172,24 @@ static int decode_operand(struct x86_emulate_ctxt *ctxt, struct operand *op,
 		fetch_register_operand(op);
 		op->orig_val = op->val;
 		break;
+	case OpAccLo:
+		op->type = OP_REG;
+		op->bytes = (ctxt->d & ByteOp) ? 2 : ctxt->op_bytes;
+		op->addr.reg = reg_rmw(ctxt, VCPU_REGS_RAX);
+		fetch_register_operand(op);
+		op->orig_val = op->val;
+		break;
+	case OpAccHi:
+		if (ctxt->d & ByteOp) {
+			op->type = OP_NONE;
+			break;
+		}
+		op->type = OP_REG;
+		op->bytes = ctxt->op_bytes;
+		op->addr.reg = reg_rmw(ctxt, VCPU_REGS_RDX);
+		fetch_register_operand(op);
+		op->orig_val = op->val;
+		break;
 	case OpDI:
 		op->type = OP_MEM;
 		op->bytes = (ctxt->d & ByteOp) ? 1 : ctxt->op_bytes;