Re: [PATCH] Add memcpy_cachebypass, a copy routine that tries to keep cache pressure down

[David Miller <davem@davemloft.net>]

From: Bryan O'Sullivan <bos@serpentine.com>
Date: Tue, 11 Jul 2006 13:50:55 -0700

> This copy routine is memcpy-compatible, but on some architectures will use
> cache-bypassing loads to avoid bringing the source data into the cache.
> 
> One case where this is useful is when a device issues a DMA to a memory
> region, and the CPU must copy the DMAed data elsewhere before doing any
> work with it.  Since the source data is read-once, write-never from the
> CPU's perspective, caching those addresses can only evict potentially
> useful data.
> 
> We provide an x86_64 implementation that uses SSE non-temporal loads,
> and a generic version that falls back to plain memcpy.
> 
> Implementors for other arches should not use cache-bypassing stores to
> the destination, as in most cases, the destination is accessed almost
> immediately after a copy finishes.
> 
> Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
> Signed-off-by: Bryan O'Sullivan <bryan.osullivan@qlogic.com>

Please don't use a weak attribute, and instead use the same
"__HAVE_ARCH_FOO" cpp test scheme used for the other string
operations to allow a platform to override the default
implementation in lib/string.x