linux-kernel - Re: [RFC PATCH 0/2] powerpc: CPU cache op cleanup

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Date:	Wed, 16 Nov 2011 10:46:32 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	"Moffett, Kyle D" <Kyle.D.Moffett@...ing.com>
Cc:	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"galak@...nel.crashing.org" <galak@...nel.crashing.org>,
	"scottwood@...escale.com" <scottwood@...escale.com>,
	"B04825@...escale.com" <B04825@...escale.com>,
	"paul.gortmaker@...driver.com" <paul.gortmaker@...driver.com>,
	Anton Blanchard <anton@....ibm.com>
Subject: Re: [RFC PATCH 0/2] powerpc: CPU cache op cleanup

On Tue, 2011-11-15 at 16:45 -0600, Moffett, Kyle D wrote:

> I guess that's doable, although I have to admit that idea almost gives
> me more of a headache than trying to fix up the 32-bit ASM.
> 
> One thing that bothers me in particular is that both 32/64 versions of
> __copy_tofrom_user() are dramatically overcomplicated for what they
> ought to be doing.
> 
> It would seem that if we get a page fault during an unaligned copy, we
> ought to just give up and fall back to a simple byte-by-byte copy loop
> from wherever we left off.  That would eliminate 90% of the ugly
> special cases without actually hurting performance, right?
> 
> For a page-fault during a cacheline-aligned copy, we should be able to
> handle the exception and retry from the last cacheline without much
> logic, again with good performance.
> 
> With that said, I'm curious about the origin of the PPC32 ASM.  In
> particular, it looks like it was generated by GCC at some point in the
> distant past, and I'm wondering if there's a good way to rewrite that
> file in C and trick GCC into generating the relevant exception tables
> for it?

There is some serious history in there :-)

I would check with Anton, he's been doing some performance work on those
lately (the 64-bit ones).

It's probably worth throwing a proof-of-concept simpler variant for
32-bit at least on the table and have people compare the perfs
(typically network perfs). I can test on a range of ppc32 here (6xx,
7xxx, 4xx).

Cheers,
Ben.


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