linux-kernel - Re: AZFS file system proposal

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Date:	Wed, 09 Jul 2008 16:48:06 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Maxim Shchetynin <maxim@...ux.vnet.ibm.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org, Paul Mackerras <paulus@...ba.org>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	Gunnar von Boehn <VONBOEHN@...ibm.com>,
	Mark Nelson <markn@....ibm.com>
Subject: Re: AZFS file system proposal


> Cc:'ing some more people that might have more of a clue on this question.
> _memcpy_fromio does a "sync" at the start and an "eieio" at the end.
> IFAICT, neither are needed here because the source is always memory.
> 
> It also handles unaligned memory accesses, which copy_to_user should
> also do correctly, so it *looks* like it should work with just a
> copy_to_user, but it still feels wrong to use an __iomem pointer
> as the source for a copy_to_user.
> 
> Any ideas?

It's a bit nasty yes. The problem is that copy_to/from_user might
do cache tricks which will blow up if the area is non-cacheable.

We have a similar problem with Mark's work on faster copy functions
since things like sys_read() can be called on userspace non-cacheable
memory such as spu local stores.

So I'm not 100% sure what the right approach here. Our copy_tofrom_user
today does dcbt on the source for example, which I hope only turns into
a no-op... The risk is if we start using dcbz.

Cheers,
Ben.


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