linux-kernel - Re: [PATCH 1/2] riscv: Add memmove string operation.

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Date:   Wed, 14 Aug 2019 11:33:13 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     Paul Walmsley <paul.walmsley@...ive.com>
CC:     Christoph Hellwig <hch@...radead.org>, nickhu@...estech.com,
        alankao@...estech.com, aou@...s.berkeley.edu, green.hu@...il.com,
        deanbo422@...il.com, tglx@...utronix.de,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        aryabinin@...tuozzo.com, glider@...gle.com, dvyukov@...gle.com,
        Anup Patel <Anup.Patel@....com>,
        Greg KH <gregkh@...uxfoundation.org>, alexios.zavras@...el.com,
        Atish Patra <Atish.Patra@....com>, zong@...estech.com,
        kasan-dev@...glegroups.com
Subject:     Re: [PATCH 1/2] riscv: Add memmove string operation.

On Tue, 13 Aug 2019 19:22:15 PDT (-0700), Paul Walmsley wrote:
> On Tue, 13 Aug 2019, Palmer Dabbelt wrote:
>
>> On Mon, 12 Aug 2019 08:04:46 PDT (-0700), Christoph Hellwig wrote:
>> > On Wed, Aug 07, 2019 at 03:19:14PM +0800, Nick Hu wrote:
>> > > There are some features which need this string operation for compilation,
>> > > like KASAN. So the purpose of this porting is for the features like KASAN
>> > > which cannot be compiled without it.
>> > >
>> > > KASAN's string operations would replace the original string operations and
>> > > call for the architecture defined string operations. Since we don't have
>> > > this in current kernel, this patch provides the implementation.
>> > >
>> > > This porting refers to the 'arch/nds32/lib/memmove.S'.
>> >
>> > This looks sensible to me, although my stringop asm is rather rusty,
>> > so just an ack and not a real review-by:
>> >
>> > Acked-by: Christoph Hellwig <hch@....de>
>>
>> FWIW, we just write this in C everywhere else and rely on the compiler to
>> unroll the loops.  I always prefer C to assembly when possible, so I'd prefer
>> if we just adopt the string code from newlib.  We have a RISC-V-specific
>> memcpy in there, but just use the generic memmove.
>>
>> Maybe the best bet here would be to adopt the newlib memcpy/memmove as generic
>> Linux functions?  They're both in C so they should be fine, and they both look
>> faster than what's in lib/string.c.  Then everyone would benefit and we don't
>> need this tricky RISC-V assembly.  Also, from the look of it the newlib code
>> is faster because the inner loop is unrolled.
>
> There's a generic memmove implementation in the kernel already:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/string.h#n362

That ends up at __builtin_memcpy(), which ends up looking for memcpy() for 
large copies, which is in lib/string.c.  The code in there is just byte at a 
time memcpy()/memmove(), which is way slower than the newlib stuff.

>
> Nick, could you tell us more about why the generic memmove() isn't
> suitable?
>
>
> - Paul