linux-kernel - [PATCH 0/2] RFC only - allow arches to over-ride unit sized user copies

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Date:	Tue, 20 Nov 2012 20:08:15 +0530
From:	<Vineet.Gupta1@...opsys.com>
To:	<arnd@...db.de>
CC:	<linux-arch@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Subject: [PATCH 0/2] RFC only - allow arches to over-ride unit sized user copies

From: Vineet Gupta <vgupta@...opsys.com>

Hi Arnd,

The current asm-generic/uaccess unit copy interface __{get,put}_user( )
defaults to using __copy_{to,from}_user( ). For archs which don't support
unaligned access, latter typically involves generated code for alignment
checks.

It is expected that arch will provide a fast path in __copy_{to,from}_user( )
for such unit sized copies - probably using __builtin_const_p( ) etc -
however the alignment checks still can't be eliminated altogether using that.
Even if they could, it woudl make the implementation messier IMHO.

However given that 2 separate interfaces already exist form beginning
(i.e. __get_user vs. __copy_from_user) will it make sense to allow arch to
provide alternate implementation of former w/o having to mess with latter.

I did a quick hack to that end and I can see that savings are more than
just "noise".

bloat-o-meter vmlinux_pre_uaccess vmlinux_uaccess_part1 | head
add/remove: 0/1 grow/shrink: 10/62 up/down: 1502/-7256 (-5754)

The patch which converts just __get_user follows. I've not
done the full/clean version yet as this is just to gather feedback.

Thx,
-Vineet

Vineet Gupta (2):
  asm-generic: uaccess: allow arch to over-ride __get_user_fn()
  ARC: provide optimal __get_user_fn()

 arch/arc/include/asm/uaccess.h |   51 ++++++++++++++++++++++++++++++++++++++++
 include/asm-generic/uaccess.h  |    5 ++++
 2 files changed, 56 insertions(+), 0 deletions(-)

-- 
1.7.4.1

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