Subject: Re: [RFC PATCH 0/2] powerpc: CPU cache op cleanup

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 16 Nov 2011 10:46:32 +1100

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.