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

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Date:   Tue, 9 Jul 2019 08:21:54 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     "Oliver O'Halloran" <oohall@...il.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 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.



> 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?

-aneesh