linux-kernel - Re: [PATCH 2/3] riscv: optimized memmove

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Date: Tue, 30 Jan 2024 21:12:03 +0800
From: Jisheng Zhang <jszhang@...nel.org>
To: Nick Kossifidis <mick@....forth.gr>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org, Matteo Croce <mcroce@...rosoft.com>,
	kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 2/3] riscv: optimized memmove

On Tue, Jan 30, 2024 at 01:39:10PM +0200, Nick Kossifidis wrote:
> On 1/28/24 13:10, Jisheng Zhang wrote:
> > From: Matteo Croce <mcroce@...rosoft.com>
> > 
> > When the destination buffer is before the source one, or when the
> > buffers doesn't overlap, it's safe to use memcpy() instead, which is
> > optimized to use a bigger data size possible.
> > 
> > Signed-off-by: Matteo Croce <mcroce@...rosoft.com>
> > Reported-by: kernel test robot <lkp@...el.com>
> > Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> 
> I'd expect to have memmove handle both fw/bw copying and then memcpy being
> an alias to memmove, to also take care when regions overlap and avoid
> undefined behavior.

Hi Nick,

Here is somthing from man memcpy:

"void *memcpy(void dest[restrict .n], const void src[restrict .n],
                    size_t n);

The  memcpy()  function copies n bytes from memory area src to memory area dest.
The memory areas must not overlap.  Use memmove(3) if the memory areas do  over‐
lap."

IMHO, the "restrict" implies that there's no overlap. If overlap
happens, the manual doesn't say what will happen.

>From another side, I have a concern: currently, other arch don't have
this alias behavior, IIUC(at least, per my understanding of arm and arm64
memcpy implementations)they just copy forward. I want to keep similar behavior
for riscv.

So I want to hear more before going towards alias-memcpy-to-memmove direction.

Thanks
> 
> 
> > --- a/arch/riscv/lib/string.c
> > +++ b/arch/riscv/lib/string.c
> > @@ -119,3 +119,28 @@ void *memcpy(void *dest, const void *src, size_t count) __weak __alias(__memcpy)
> >   EXPORT_SYMBOL(memcpy);
> >   void *__pi_memcpy(void *dest, const void *src, size_t count) __alias(__memcpy);
> >   void *__pi___memcpy(void *dest, const void *src, size_t count) __alias(__memcpy);
> > +
> > +/*
> > + * Simply check if the buffer overlaps an call memcpy() in case,
> > + * otherwise do a simple one byte at time backward copy.
> > + */
> > +void *__memmove(void *dest, const void *src, size_t count)
> > +{
> > +	if (dest < src || src + count <= dest)
> > +		return __memcpy(dest, src, count);
> > +
> > +	if (dest > src) {
> > +		const char *s = src + count;
> > +		char *tmp = dest + count;
> > +
> > +		while (count--)
> > +			*--tmp = *--s;
> > +	}
> > +	return dest;
> > +}
> > +EXPORT_SYMBOL(__memmove);
> > +
> 
> Here is an approach for the backwards case to get things started...
> 
> static void
> copy_bw(void *dst_ptr, const void *src_ptr, size_t len)
> {
> 	union const_data src = { .as_bytes = src_ptr + len };
> 	union data dst = { .as_bytes = dst_ptr + len };
> 	size_t remaining = len;
> 	size_t src_offt = 0;
> 
> 	if (len < 2 * WORD_SIZE)
> 		goto trailing_bw;
> 
> 	for(; dst.as_uptr & WORD_MASK; remaining--)
> 		*--dst.as_bytes = *--src.as_bytes;
> 
> 	src_offt = src.as_uptr & WORD_MASK;
> 	if (!src_offt) {
> 		for (; remaining >= WORD_SIZE; remaining -= WORD_SIZE)
> 			*--dst.as_ulong = *--src.as_ulong;
> 	} else {
> 		unsigned long cur, prev;
> 		src.as_bytes -= src_offt;
> 		for (; remaining >= WORD_SIZE; remaining -= WORD_SIZE) {
> 			cur = *src.as_ulong;
> 			prev = *--src.as_ulong;
> 			*--dst.as_ulong = cur << ((WORD_SIZE - src_offt) * 8) |
> 					  prev >> (src_offt * 8);
> 		}
> 		src.as_bytes += src_offt;
> 	}
> 
>  trailing_bw:
> 	while (remaining-- > 0)
> 		*--dst.as_bytes = *--src.as_bytes;
> }
> 
> Regards,
> Nick