Re: Changes in s390x uaccess.h in 2.4.17+

["Ulrich Weigand" <Ulrich.Weigand@de.ibm.com>]

Pete Zaitcev wrote:

>The 2.4.17 patch comes with a change to uaccess.h, which
>contains the following:
>
>- * These functions have a non-standard call interface
>+ * These functions have standard call interface
>
>Does it look familiar to anyone? The change is in Marcelo's
>tree now, but I am curious what it actually does.

This is just a minor optimization.  The old code used to
have the core of __copy_to_user_asm etc. inline, and only
the exception fixup code in lib (__copy_to_user_fixup).

The point of this was to save the cost of a function call
for the default case.  However, because of various reasons
(tedious argument reloading, register pressure, code size)
this 'optimization' turned out to be actually worse than
just a plain function call, so we threw it out. The new code
simply implements __copy_to_user_asm completely in lib.

>The 2.4.17 patch readme says "This patch contains the current
>recommended kernel 2.4.7 code base (up to 2002-01-21),
>adapted to kernel 2.4.17.", but I cannot find this change
>in any published 2.4.7 patches. I am pretty sure there must
>be a good reason for the change, and it's interesting to
>know what it is.

The 2.4.17 patch also contains a number of minor cleanups
that should not cause any noticeable difference in behaviour,
like this one.

We did not find it worthwhile to backport this to 2.4.7;
we usually do that only for bugfixes.


Mit freundlichen Gruessen / Best Regards

Ulrich Weigand

--
  Dr. Ulrich Weigand
  Linux for S/390 Design & Development
  IBM Deutschland Entwicklung GmbH, Schoenaicher Str. 220, 71032 Boeblingen
  Phone: +49-7031/16-3727   ---   Email: Ulrich.Weigand@de.ibm.com