From: Bernhard Beschow <shentey@gmail.com>

Compared to decode_instruction(), decode_instruction_stream() has an additional
stream parameter which avoids some guest memory accesses during instruction
decoding. Both functions defer the actual work to decode_opcode() which would
set the stream pointer to zero such that decode_instruction_stream() essentially
behaved like decode_instruction(). Given that all callers of
decode_instruction_stream() properly zero-initialize the decode parameter, the
memset() call can be moved into decode_instruction() which is the only other
user of decode_opcode(). This preserves the non-zero stream pointer which
avoids extra guest memory accesses.

Fixes: 1e25327b244a ("target/i386/emulate: Allow instruction decoding from stream")
cc: qemu-stable
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
---
 target/i386/emulate/x86_decode.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/target/i386/emulate/x86_decode.c b/target/i386/emulate/x86_decode.c
index 6ad03b71b0..7bbcd2a9a2 100644
--- a/target/i386/emulate/x86_decode.c
+++ b/target/i386/emulate/x86_decode.c
@@ -2088,8 +2088,6 @@ static void decode_opcodes(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);
@@ -2101,6 +2099,8 @@ static uint32_t decode_opcode(CPUX86State *env, struct x86_decode *decode)
 
 uint32_t decode_instruction(CPUX86State *env, struct x86_decode *decode)
 {
+    memset(decode, 0, sizeof(*decode));
+
     return decode_opcode(env, decode);
 }
 
-- 
2.50.1 (Apple Git-155)

> Fixes: 1e25327b244a ("target/i386/emulate: Allow instruction decoding from stream")
> cc: qemu-stable
> Signed-off-by: Bernhard Beschow <shentey@gmail.com>
> Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
> ---
>  target/i386/emulate/x86_decode.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/target/i386/emulate/x86_decode.c b/target/i386/emulate/x86_decode.c
> index 6ad03b71b0..7bbcd2a9a2 100644
> --- a/target/i386/emulate/x86_decode.c
> +++ b/target/i386/emulate/x86_decode.c
> @@ -2088,8 +2088,6 @@ static void decode_opcodes(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);
> @@ -2101,6 +2099,8 @@ static uint32_t decode_opcode(CPUX86State *env, struct x86_decode *decode)
>  
>  uint32_t decode_instruction(CPUX86State *env, struct x86_decode *decode)
>  {
> +    memset(decode, 0, sizeof(*decode));
> +
>      return decode_opcode(env, decode);
>  }
>  
> -- 
> 2.50.1 (Apple Git-155)

Tested-by: Magnus Kulke <magnuskulke@linux.microsoft.com>

makes sense, thx.