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>
Reviewed-by: Wei Liu (Microsoft) <wei.liu@kernel.org>
Tested-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
Link: https://lore.kernel.org/r/20260223233950.96076-4-mohamed@unpredictable.fr
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 1b93832f55927b1b76a6587ca75a5a35676188de)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

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);
 }
 
-- 
2.47.3