Re: [Qemu-devel] [PATCH 07/19] target/ppc: Make special ORs match x86 pause and don't generate on mttcg

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2019-02-12 at 16:59 +1100, David Gibson wrote:
> On Mon, Jan 28, 2019 at 10:46:13AM +0100, Cédric Le Goater wrote:
> > From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > 
> > There's no point in going out of translation on an SMT OR with
> > mttcg since the backend won't do anything useful such as pausing,
> > it's only useful on traditional TCG to give time to other
> > processors.
> 
> Is it actively harmful in the MTTCG case, or just pointless?

I think it can hurt performance, I don't remember for sure :)

> > Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Signed-off-by: Cédric Le Goater <clg@kaod.org>
> > ---
> >  target/ppc/translate.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/target/ppc/translate.c b/target/ppc/translate.c
> > index e169c43643a1..7d40a1fbe6bd 100644
> > --- a/target/ppc/translate.c
> > +++ b/target/ppc/translate.c
> > @@ -1580,7 +1580,7 @@ static void gen_pause(DisasContext *ctx)
> >      tcg_temp_free_i32(t0);
> >  
> >      /* Stop translation, this gives other CPUs a chance to run */
> > -    gen_exception_nip(ctx, EXCP_HLT, ctx->base.pc_next);
> > +    gen_exception_nip(ctx, EXCP_INTERRUPT, ctx->base.pc_next);
> 
> I don't see how this change relates to the rest.

Yeah not sure anymore :-)

> >  }
> >  #endif /* defined(TARGET_PPC64) */
> >  
> > @@ -1662,7 +1662,9 @@ static void gen_or(DisasContext *ctx)
> >           * than no-op, e.g., miso(rs=26), yield(27), mdoio(29), mdoom(30),
> >           * and all currently undefined.
> >           */
> > -        gen_pause(ctx);
> > +        if (!mttcg_enabled) {
> > +            gen_pause(ctx);
> > +        }
> >  #endif
> >  #endif
> >      }