target/i386/emulate: Allow instruction decoding from stream

Introduce a new helper function to decode x86 instructions from a
raw instruction byte stream. MSHV delivers an instruction stream in a
buffer of the vm_exit message. It can be used to speed up MMIO
emulation, since instructions do not have to be fetched and translated.

Added "fetch_instruction()" op to x86_emul_ops() to improve
traceability.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250916164847.77883-3-magnuskulke@linux.microsoft.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/emulate/x86_decode.c

@@ -71,10 +71,16 @@ static inline uint64_t decode_bytes(CPUX86State *env, struct x86_decode *decode,
         VM_PANIC_EX("%s invalid size %d\n", __func__, size);
         break;
     }
-    target_ulong va  = linear_rip(env_cpu(env), env->eip) + decode->len;
-    emul_ops->read_mem(env_cpu(env), &val, va, size);
+
+    /* copy the bytes from the instruction stream, if available */
+    if (decode->stream && decode->len + size <= decode->stream->len) {
+        memcpy(&val, decode->stream->bytes + decode->len, size);
+    } else {
+        target_ulong va = linear_rip(env_cpu(env), env->eip) + decode->len;
+        emul_ops->fetch_instruction(env_cpu(env), &val, va, size);
+    }
     decode->len += size;
-    
+
     return val;
 }
 
@@ -2076,9 +2082,10 @@ static void decode_opcodes(CPUX86State *env, struct x86_decode *decode)
     }
 }
 
-uint32_t decode_instruction(CPUX86State *env, struct x86_decode *decode)
+static uint32_t decode_opcode(CPUX86State *env, struct x86_decode *decode)
 {
     memset(decode, 0, sizeof(*decode));
+
     decode_prefix(env, decode);
     set_addressing_size(env, decode);
     set_operand_size(env, decode);
@@ -2088,6 +2095,18 @@ uint32_t decode_instruction(CPUX86State *env, struct x86_decode *decode)
     return decode->len;
 }
 
+uint32_t decode_instruction(CPUX86State *env, struct x86_decode *decode)
+{
+    return decode_opcode(env, decode);
+}
+
+uint32_t decode_instruction_stream(CPUX86State *env, struct x86_decode *decode,
+                                   struct x86_insn_stream *stream)
+{
+    decode->stream = stream;
+    return decode_opcode(env, decode);
+}
+
 void init_decoder(void)
 {
     int i;

target/i386/emulate/x86_decode.h

@@ -272,6 +272,11 @@ typedef struct x86_decode_op {
     };
 } x86_decode_op;
 
+typedef struct x86_insn_stream {
+    const uint8_t *bytes;
+    size_t len;
+} x86_insn_stream;
+
 typedef struct x86_decode {
     int len;
     uint8_t opcode[4];
@@ -298,11 +303,15 @@ typedef struct x86_decode {
     struct x86_modrm modrm;
     struct x86_decode_op op[4];
     bool is_fpu;
+
+    x86_insn_stream *stream;
 } x86_decode;
 
 uint64_t sign(uint64_t val, int size);
 
 uint32_t decode_instruction(CPUX86State *env, struct x86_decode *decode);
+uint32_t decode_instruction_stream(CPUX86State *env, struct x86_decode *decode,
+                                   struct x86_insn_stream *stream);
 
 void *get_reg_ref(CPUX86State *env, int reg, int rex_present,
                   int is_extended, int size);

target/i386/emulate/x86_emu.c

@@ -1246,7 +1246,8 @@ static void init_cmd_handler(void)
 bool exec_instruction(CPUX86State *env, struct x86_decode *ins)
 {
     if (!_cmd_handler[ins->cmd].handler) {
-        printf("Unimplemented handler (" TARGET_FMT_lx ") for %d (%x %x) \n", env->eip,
+        printf("Unimplemented handler (" TARGET_FMT_lx ") for %d (%x %x)\n",
+                env->eip,
                 ins->cmd, ins->opcode[0],
                 ins->opcode_len > 1 ? ins->opcode[1] : 0);
         env->eip += ins->len;

target/i386/emulate/x86_emu.h

@@ -24,6 +24,8 @@
 #include "cpu.h"
 
 struct x86_emul_ops {
+    void (*fetch_instruction)(CPUState *cpu, void *data, target_ulong addr,
+                              int bytes);
     void (*read_mem)(CPUState *cpu, void *data, target_ulong addr, int bytes);
     void (*write_mem)(CPUState *cpu, void *data, target_ulong addr, int bytes);
     void (*read_segment_descriptor)(CPUState *cpu, struct x86_segment_descriptor *desc,