linux-kernel - Re: [PATCH 4/4] powerpc/64: reuse PPC32 static inline flush_dcache

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Date:   Tue, 9 Jul 2019 12:20:12 +1000
From:   "Oliver O'Halloran" <oohall@...il.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] powerpc/64: reuse PPC32 static inline flush_dcache_range()

On Tue, Jul 9, 2019 at 12:22 AM Aneesh Kumar K.V
<aneesh.kumar@...ux.ibm.com> wrote:
>
> Christophe Leroy <christophe.leroy@....fr> writes:
>
> > *snip*
> > +     if (IS_ENABLED(CONFIG_PPC64))
> > +             isync();
> >  }
>
>
> Was checking with Michael about why we need that extra isync. Michael
> pointed this came via
>
> https://github.com/mpe/linux-fullhistory/commit/faa5ee3743ff9b6df9f9a03600e34fdae596cfb2#diff-67c7ffa8e420c7d4206cae4a9e888e14
>
> for 970 which doesn't have coherent icache. So possibly isync there is
> to flush the prefetch instructions? But even so we would need an icbi
> there before that isync.

I don't think it's that, there's some magic in flush_icache_range() to
handle dropping prefetched instructions on 970.

> So overall wondering why we need that extra barriers there.

I think the isync is needed there because the architecture only
requires sync to provide ordering. A sync alone doesn't guarantee the
dcbfs have actually completed so the isync is necessary to ensure the
flushed cache lines are back in memory. That said, as far as I know
all the IBM book3s chips from power4 onwards will wait for pending
dcbfs when they hit a sync, but that might change in the future.

If it's a problem we could add a cpu-feature section around the isync
to no-op it in the common case. However, when I had a look with perf
it always showed that the sync was the hotspot so I don't think it'll
help much.

Oliver