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Date: Sat, 16 Nov 2002 19:30:03 +0100
From: Andi Kleen <ak@suse.de>
To: Akira Tsukamoto <at541@columbia.edu>
Cc: Andi Kleen <ak@suse.de>, linux-kernel@vger.kernel.org,
       Hirokazu Takahashi <taka@valinux.co.jp>, Andrew Morton <akpm@digeo.com>,
       Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua>
Subject: Re: [CFT][PATCH]  2.5.47 Athlon/Druon, much faster copy_user function
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References: <20021115235234.8DE4.AT541@columbia.edu> <20021116115652.A26519@wotan.suse.de> <20021116131403.9FB5.AT541@columbia.edu>
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On Sat, Nov 16, 2002 at 01:22:51PM -0500, Akira Tsukamoto wrote:
> On Sat, 16 Nov 2002 11:56:52 +0100
> Andi Kleen <ak@suse.de> mentioned:
> > 
> > You don't seem to save/restore the FPU state, so it will be likely 
> > corrupted after your copy runs.
> 
> This is the main question for me that I was wondering for all week. 
> My first version was using fsave and frstore, so 
> just changing three lines will accomplish this.
> Is it all I need?  Any thing elase needed to consider using fpu register?

You are currently corrupting the user's FPU state.

The proper way to save it is to use kernel_fpu_begin()

> > 
> > Also I'm pretty sure that using movntq (= forcing destination out of 
> > cache) is not a good strategy for generic copy_from_user(). It may 
> > be a win for the copies in write ( user space -> page cache ),
> 
> Yes, that why I included postfetch in the code because movntq does not leave 
> them in the L2 cache.

That looks rather wasteful - first force it out and then trying to get it in 
again. I have my doubts on it being a good strategy for speed.

> > but 
> > will hurt for all the ioctls and other things that actually need the
> > data in cache afterwards. I am afraid it is not enough to do micro benchmarks
> > here.
> 
> check above?

Use special function calls for them, don't put it into generic 
copy_*_user

Also you should really check for small copy and not use the FPU based
copy for them. Best is probably to use a relatively simply copy_*_user
(no FPU tricks, just an unrolled integer core) and change the VFS
and the file systems to call a special function from write(), but only
when the write is big.

-Andi
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