linux-kernel - Re: [PATCH 3/3] Fix copy_user on x86

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Date:	Mon, 30 Jun 2008 17:12:10 +0200
From:	Vitaly Mayatskikh <v.mayatskih@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Vitaly Mayatskikh <v.mayatskih@...il.com>,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] Fix copy_user on x86_64

Linus Torvalds <torvalds@...ux-foundation.org> writes:

>> Added copy_user_64.c instead of copy_user_64.S and
>> copy_user_nocache_64.S
>
> Grr, your patches are as attachements, which means that answering to 
> themmakes quoting them much harder. 

Sorry... But what was mentioned in Documentation/SubmittingPatches with:

"For this reason, all patches should be submitting e-mail "inline".
WARNING:  Be wary of your editor's word-wrap corrupting your patch,
if you choose to cut-n-paste your patch."

My first thought was "should be attached inline".

> Hmm? Sorry for being such a stickler. This code does end up being fairly 
> critical, both for correctness and for performance. A lot of user copies 
> are actually short (smallish structures, or just small reads and writes), 
> and I'd like to make sure that really basic infrastructure like this is 
> just basically "known to be optimal", so that we can totally forget about 
> it for a few more years.

Agreed. Code was reworked again, will test it a bit more. Two more
questions to you and Andi:

1. Do you see any reasons to do fix alignment for destination as it was
done in copy_user_generic_unrolled (yes, I know, access to unaligned
address is slower)? It tries to byte-copy unaligned bytes first and then
to do a normal copy. I think, most times destination addresses will be
aligned and this check is not so necessary. If it is necessary, then
copy_user_generic_string should do the same.

2. What is the purpose of "minor optimization" in commit
3022d734a54cbd2b65eea9a024564821101b4a9a?

ENTRY(copy_user_generic_string)
        CFI_STARTPROC
        movl %ecx,%r8d          /* save zero flag */
        movl %edx,%ecx
        shrl $3,%ecx
        andl $7,%edx
        jz   10f
1:      rep
        movsq
        movl %edx,%ecx
2:      rep
        movsb
9:      movl %ecx,%eax
        ret

        /* multiple of 8 byte */
10:     rep
        movsq
        xor %eax,%eax
        ret

I don't think CPU is able to speculate with 'rep movs*' in both
branches, and I'm not sure if conditional jump is cheaper then empty
'rep movsb' (when ecx is 0). I want to eliminate it if you don't have
any objections.

Thanks.
-- 
wbr, Vitaly
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