hv: Fix two minor issues in instruction emulation code

1. The wrong operand size is assigned in instruction decode phase
   if the operand size is 1 byte.

   According to the SDM, the bit 0(w bit) of opcode should be checked
   first to detect whether the operand size is 1 byte. Then, check
   whether there is prefix to overwrite the default operand size.

   The original instruction decode doesn't care about the operand
   size. But do opsize fixup during instruction emulation phase.
   With ACRN we need operand size packed to ioreq and send to DM
   after instruction decode.

2. We should always touch the GPA by following opsize to avoid side
   effect (especially when GPA is for a MMIO).

Tracked-On: #1337
Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
Acked-by: Anthony Xu <anthony.xu@intel.com>

Author: fyin1

hypervisor/arch/x86/guest/instr_emul.c

@@ -982,16 +982,15 @@ static int get_gva_di_check(struct vcpu *vcpu, struct instr_emul_vie *vie,
  */
 static int emulate_movs(struct vcpu *vcpu, struct instr_emul_vie *vie)
 {
-	uint64_t src_gva, gpa, val;
+	uint64_t src_gva, gpa, val = 0UL;
 	uint64_t *dst_hva, *src_hva;
 	uint64_t rcx, rdi, rsi, rflags;
 	uint32_t err_code;
 	enum cpu_reg_name seg;
 	int error, repeat;
-	uint8_t opsize;
+	uint8_t opsize = vie->opsize;
 	bool is_mmio_write;
 
-	opsize = (vie->opcode == 0xA4U) ? 1U : vie->opsize;
 	error = 0;
 	is_mmio_write = (vcpu->req.reqs.mmio.direction == REQUEST_WRITE);
 
@@ -1025,8 +1024,7 @@ static int emulate_movs(struct vcpu *vcpu, struct instr_emul_vie *vie)
 		/* we are sure it will success */
 		(void)gva2gpa(vcpu, src_gva, &gpa, &err_code);
 		src_hva = gpa2hva(vcpu->vm, gpa);
-
-		val = *src_hva;
+		(void)memcpy_s(&val, opsize, src_hva, opsize);
 
 		mmio_write(vcpu, val);
 	} else {
@@ -1036,7 +1034,7 @@ static int emulate_movs(struct vcpu *vcpu, struct instr_emul_vie *vie)
 		 * decoding.
 		 */
 		dst_hva = gpa2hva(vcpu->vm, vie->dst_gpa);
-		memcpy_s(dst_hva, opsize, &val, opsize);
+		(void)memcpy_s(dst_hva, opsize, &val, opsize);
 	}
 
 	rsi = vm_get_register(vcpu, CPU_REG_RSI);
@@ -1072,11 +1070,10 @@ static int emulate_movs(struct vcpu *vcpu, struct instr_emul_vie *vie)
 static int emulate_stos(struct vcpu *vcpu, struct instr_emul_vie *vie)
 {
 	int error, repeat;
-	uint8_t opsize;
+	uint8_t opsize = vie->opsize;
 	uint64_t val;
 	uint64_t rcx, rdi, rflags;
 
-	opsize = (vie->opcode == 0xAAU) ? 1U : vie->opsize;
 	repeat = vie->repz_present | vie->repnz_present;
 
 	if (repeat != 0) {
@@ -1800,11 +1797,13 @@ static int decode_two_byte_opcode(struct instr_emul_vie *vie)
 	}
 
 	vie_advance(vie);
+
 	return 0;
 }
 
 static int decode_opcode(struct instr_emul_vie *vie)
 {
+	int ret = 0;
 	uint8_t x;
 
 	if (vie_peek(vie, &x) != 0) {
@@ -1821,10 +1820,20 @@ static int decode_opcode(struct instr_emul_vie *vie)
 	vie_advance(vie);
 
 	if (vie->op.op_type == VIE_OP_TYPE_TWO_BYTE) {
-		return decode_two_byte_opcode(vie);
+		ret = decode_two_byte_opcode(vie);
 	}
 
-	return 0;
+	/* Fixup the opsize according to opcode w bit:
+	 * If w bit of opcode is 0, the operand size is 1 byte
+	 * If w bit of opcode is 1, the operand size is decided
+	 * by prefix and default operand size attribute (handled
+	 * in decode_prefixes).
+	 */
+	if ((ret == 0) && ((vie->opcode & 0x1U) == 0U)) {
+		vie->opsize = 1U;
+	}
+
+	return ret;
 }
 
 static int decode_modrm(struct instr_emul_vie *vie, enum vm_cpu_mode cpu_mode)