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

[Richard Henderson]

On 06/21/2016 12:28 AM, Paolo Bonzini wrote:
> On 18/06/2016 06:03, Pranith Kumar wrote:
> > Signed-off-by: Pranith Kumar <address@hidden>
> > ---
> >  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.

They're not unnecessary if we (1) add those barriers for normal loads and 
stores and (2) omit them from the non-temporal loads and stores.

In the meantime they're good documentation.


r~