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

[David Gibson]

On Wed, Feb 13, 2019 at 11:03:12AM +1100, Benjamin Herrenschmidt wrote:
> 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 <address@hidden>
> > > 
> > > 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 <address@hidden>
> > > Signed-off-by: Cédric Le Goater <address@hidden>
> > > ---
> > >  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 :-)

Oh.  That certainly doesn't make this easier to review.

So, all these target/ppc patches are only indirectly related to XIVE
pnv support.  Cédric, can you split them out into their own series on
the next spin.

> 
> > >  }
> > >  #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
> > >      }
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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