[PATCH] ARM: optimize memset_io()/memcpy_fromio()/memcpy_toio()

[Will Deacon]

On Fri, Sep 28, 2012 at 05:17:53AM +0100, Nicolas Pitre wrote:
> On Thu, 27 Sep 2012, Russell King wrote:
> 
> > If we are building for a LE platform, and we haven't overriden the
> > MMIO ops, then we can optimize the mem*io operations using the
> > standard string functions.
> > 
> > Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
> 
> We presume that the IO space is able to cope with a mixture of access 
> width other than byte access which should be perfectly reasonable by 
> default.  If so then...
> 
> Acked-by: Nicolas Pitre <nico at .linaro.org>

This looks pretty scary to me, but maybe I'm worrying too much. The first
thing to ensure is that the accesses are always aligned, which I believe is
true for the string operations. However, a quick glance at memset shows that
we do things like store multiple with writeback addressing modes. This is
bad for a few reasons:

	1. If an access other the first one generated by the instruction
	   causes an abort, the CPU will ultimately re-execute the earlier
	   accesses, which could be problematic to a device.

	2. Writeback addressing modes when accessing MMIO peripherals causes
	   serious performance problems with virtualisation, as I have
	   described before.

	3. We have to guarantee that no single instruction causes accesses
	   that span a page boundary, as this leads to UNPREDICTABLE
	   behaviour.

So, unless we can guarantee that our accesses are all aligned, will never
fault, do not cross a page boundary and we are not running as a guest then
I'd be inclined to stick with byte-by-byte implementations for these
functions.

Will