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

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Date:   Tue, 9 Jul 2019 15:29:43 +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 12:52 PM Aneesh Kumar K.V
<aneesh.kumar@...ux.ibm.com> wrote:
>
> On 7/9/19 7:50 AM, Oliver O'Halloran wrote:
> > 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.
> >
>
> ISA doesn't list that as the sequence. Only place where isync was
> mentioned was w.r.t  icbi where want to discards the prefetch.

doesn't list that as the sequence for what?

> > 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.
> >
>
> What about the preceding barriers (sync; isync;) before dcbf? Why are
> they needed?

Dunno, the sync might just be to ensure ordering between prior stores
and the dcbf.

>
> -aneesh