lkml.org 
[lkml]   [2008]   [Jul]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/2] Introduce copy_user_handle_tail routine
Vitaly Mayatskikh wrote:
> Andi Kleen <andi@firstfloor.org> writes:
>
>>>>>> rep movs can fail.
>>>> How? (if it's a byte copy?)
>>> Parameter len is a number of uncopied bytes,
>> But that is exactly what copy_*_user wants to return
>
> Last experience showed, it doesn't.

?

> Ok, when unrolled version fails on reading quad word at unaligned
> address, it doesn't know where it was failed exactly. At this moment it
> hasn't correct number of uncopied bytes, because some bytes can still
> remain at the very end of the page. copy_user_handle_tail copies them
> and return correct value on uncopied bytes. Complicated logic for
> counting the number of these bytes is not necessary to optimize at
> assembly level, because we already missed performance. It's hard to
> complain against it.

Yes I'm talking about the "replay loop"

There's no complicated logic in a rep ; movs. And it's still
a byte copy. In fact it is far simpler than what you already have.

>> The original version I wrote returned "unfaulted bytes" which was wrong.
>> Correct is "uncopied" as fixed by Linus. rep ; movs returns uncopied.
>
> It's not in C. If you have the proposal why it should be written in
> assembly, send it to Linus.

Well it would turn your 15+ lines C function in ~4 (well tested) lines or so.

>
>>> Why do you think that zeroing can never fail, even in userspace?
>> There's no zeroing in user space, only in kernel space.
>
> Agree.
>
>> The only reason kernel does it is to avoid leaking uninitialized data,
>> but for user space it doesn't make sense (see above)
>
> Ok, copy_in_user can pass zerorest=0 to copy_user_handle_tail. Is it ok
> for you?

My point was that for the zeroing you can just use memset(), there's
no need to support faulting there at all.

-Andi



\
 
 \ /
  Last update: 2008-07-02 20:57    [W:0.117 / U:0.108 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site