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

[Paul Walmsley]

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@xxxxxx>
> 
> 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

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


- Paul