Re: [Qemu-devel] [RFC v3 PATCH 14/14] target-i386: Generate fences for x86

[Paolo Bonzini <pbonzini@redhat.com>]

On 18/06/2016 06:03, Pranith Kumar wrote:
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> ---
>  target-i386/translate.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/target-i386/translate.c b/target-i386/translate.c
> index bf33e6b..32b0f5c 100644
> --- a/target-i386/translate.c
> +++ b/target-i386/translate.c
> @@ -8012,13 +8012,17 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
>                  || (prefixes & PREFIX_LOCK)) {
>                  goto illegal_op;
>              }
> +            tcg_gen_mb(TCG_MO_ST_ST | TCG_BAR_SC);
>              break;
>          case 0xe8 ... 0xef: /* lfence */
> +            tcg_gen_mb(TCG_MO_LD_LD | TCG_BAR_SC);
> +            break;

These are unnecessary.  On the other hand, _each and every load_ must be
followed by a LD_LD | LD_ST barrier, and each and every store must be
preceded by a LD_ST | ST_ST barrier.

Paolo

>          case 0xf0 ... 0xf7: /* mfence */
>              if (!(s->cpuid_features & CPUID_SSE2)
>                  || (prefixes & PREFIX_LOCK)) {
>                  goto illegal_op;
>              }
> +            tcg_gen_mb(TCG_MO_ALL | TCG_BAR_SC);
>              break;
>  
>          default:
>