tcg: Increase the highwater reservation

If there are a lot of guest memory ops in the TB, the amount of
code generated by tcg_out_tb_finalize could be well more than 1k.
In the short term, increase the reservation larger than any TB
seen in practice.
Reported-by: NAurelien Jarno <aurelien@aurel32.net>
Reviewed-by: NAurelien Jarno <aurelien@aurel32.net>
Tested-by: NAurelien Jarno <aurelien@aurel32.net>
Signed-off-by: NRichard Henderson <rth@twiddle.net>

tcg/tcg.c

@@ -388,7 +388,11 @@ void tcg_prologue_init(TCGContext *s)
     /* Compute a high-water mark, at which we voluntarily flush the buffer
        and start over.  The size here is arbitrary, significantly larger
        than we expect the code generation for any one opcode to require.  */
-    s->code_gen_highwater = s->code_gen_buffer + (total_size - 1024);
+    /* ??? We currently have no good estimate for, or checks in,
+       tcg_out_tb_finalize.  If there are quite a lot of guest memory ops,
+       the number of out-of-line fragments could be quite high.  In the
+       short-term, increase the highwater buffer.  */
+    s->code_gen_highwater = s->code_gen_buffer + (total_size - 64*1024);
 
     tcg_register_jit(s->code_gen_buffer, total_size);