From mboxrd@z Thu Jan  1 00:00:00 1970
From: catalin.marinas@arm.com (Catalin Marinas)
Date: Thu, 28 Jan 2010 13:31:01 +0000
Subject: [PATCH] ARM: Add SWP/SWPB emulation for ARMv7 processors (v4)
In-Reply-To: <20100114130700.27721.63614.stgit@e101986-lin>
References: <20100114130700.27721.63614.stgit@e101986-lin>
Message-ID: <1264685461.7064.65.camel@pc1117.cambridge.arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Thu, 2010-01-14 at 13:07 +0000, Leif Lindholm wrote:
> The SWP instruction was deprecated in the ARMv6 architecture, superseded
> by the LDREX/STREX family of instructions for
> load-linked/store-conditional operations. The ARMv7 multiprocessing
> extensions mandate that SWP/SWPB instructions are treated as undefined
> from reset, with the ability to enable them through the System Control
> Register SW bit.
> 
> This patch adds the alternative solution to emulate the SWP and SWPB
> instructions using LDREX/STREX sequences, and log statistics to
> /proc/cpu/swp_emulation. To correctly deal with copy-on-write, it also
> modifies cpu_v7_set_pte_ext to change the mappings to priviliged RO when
> user RO.

Just a thought - could we not add another kuser helper in the vectors
page that does this? Your exception trapping code would set regs->ARM_lr
to the next instruction following SWP and regs->ARM_pc to the helper. 

This way you don't have to worry about handling another exception from
inside the kernel or modifying set_pte_ext.

-- 
Catalin