Re: [PATCH] x86: cache pollution aware __copy_from_user_ll()

[Linus Torvalds]

On Sat, 24 Jun 2006, Arjan van de Ven wrote:
> 
> while this patch will reduce the number of cycles spent in the kernel,
> it's just pushing the cache miss to userspace (by virtue of doing a
> cache flush effectively)... is this really the right thing? The total
> memory bandwidth will actually increase with this patch if you're
> unlucky (eg if userspace decides to write to this memory eventually)....

No. It's for copying _from_ user space, ie a "write()" system call. So 
what it does is to effectively try to use non-temporal stores to the page 
cache - since the page cache is usually not read directly afterwards (at 
least not soon enough for L1 caches to help).

I don't generally like cache tricks either (caches tend to be better than 
humans, or at least get there fairly soon), but this one does seem very 
valid.

		Linus
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