linux-kernel - Re: [RFC PATCH] powerpc: Convert ____flush_dcache_icache

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Message-ID: <c22dfd19-872a-afe1-40c6-82e11b0af9e1@c-s.fr>
Date:   Thu, 22 Aug 2019 07:06:03 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Alastair D'Silva <alastair@....ibm.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH] powerpc: Convert ____flush_dcache_icache_phys() to C



Le 22/08/2019 à 02:27, Alastair D'Silva a écrit :
> On Wed, 2019-08-21 at 22:27 +0200, Christophe Leroy wrote:
>>
>> Le 20/08/2019 à 06:36, Alastair D'Silva a écrit :
>>> On Fri, 2019-08-16 at 15:52 +0000, Christophe Leroy wrote:
>>
>> [...]
>>
>>>
>>> Thanks Christophe,
>>>
>>> I'm trying a somewhat different approach that requires less
>>> knowledge
>>> of assembler. Handling of CPU_FTR_COHERENT_ICACHE is outside this
>>> function. The code below is not a patch as my tree is a bit messy,
>>> sorry:
>>
>> Can we be 100% sure that GCC won't add any code accessing some
>> global
>> data or stack while the Data MMU is OFF ?
>>
>> Christophe
>>
> 
> +mpe
> 
> I'm not sure how we would go about making such a guarantee, but I've
> tied every variable used to a register and addr is passed in a
> register, so there is no stack usage, and every call in there only
> operates on it's operands.
> 
> The calls to the inline cache helpers (for the PPC32 case) are all
> constants, so I can't see a reasonable scenario where there would be a
> function call and reordered to after the DR bit is turned off, but I
> guess if we want to be paranoid, we could always add an mb() call
> before the DR bit is manipulated to prevent the compiler from
> reordering across the section where the data MMU is disabled.
> 
> 

Anyway, I think the benefit of converting that function to C is pretty 
small. flush_dcache_range() and friends were converted to C mainly in 
order to inline them. But this __flush_dcache_icache_phys() is too big 
to be worth inlining, yet small and stable enough to remain in assembly 
for the time being.

So I suggest you keep it aside your series for now, just move 
PURGE_PREFETCHED_INS inside it directly as it will be the only remaining 
user of it.

Christophe