linux-kernel - Re: [patch] x86, mm: pass in 'total' to __copy_from_user

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Date:	Sat, 28 Feb 2009 16:06:51 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	torvalds@...ux-foundation.org
Cc:	arjan@...radead.org, mingo@...e.hu, nickpiggin@...oo.com.au,
	sqazi@...gle.com, linux-kernel@...r.kernel.org, tglx@...utronix.de,
	hpa@...or.com, andi@...stfloor.org
Subject: Re: [patch] x86, mm: pass in 'total' to __copy_from_user_*nocache()

From: Linus Torvalds <torvalds@...ux-foundation.org>
Date: Sat, 28 Feb 2009 09:42:18 -0800 (PST)

> On Sat, 28 Feb 2009, Arjan van de Ven wrote:
> > 
> > it invalidates all caches in the hierarchy
> 
> Yeah, now that I look at the intel pdf's, I see that.
> 
> > afaik this is what Intel cpus do; but I also thought this behavior was
> > quite architectural as well...
> 
> Ok, I really think we should definitely not use non-temporal stores for 
> anything smaller than one full page in that case. In fact, I wonder if 
> even any of the old streaming benchmarks are even true. I thought it would 
> still stay in the L3, but yes, it literally seems to make the access 
> totally noncached and WC.
> 
> That's almost unacceptable in the long run. With a 8MB L3 cache - and a 
> compile sequence, do we really want to go out to memory to write the .S 
> file, and then have the assembler go out to memory to read it back? For a 
> compile, that _probably_ is all fine (the compiler in particular will have 
> enough data structures around that it's not going to fit in the cache 
> anyway), but I'm seeing leaner compilers and other cases where forcing 
> things out all the way on the bus is simply the wrong thing.

I think this is an accurate analysis as well, it's really unfortunate
the non-temporal stuff on x86 doesn't preserve existing cache lines
when present.

I thought that was the whole point.  Don't pollute the caches, but
if cache lines are already loaded there, use them and don't purge!
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