Re: [PATCH 01/31] MIPS: Move allocate_kscratch to cpu-probe.c andmake it public.

From: Ralf Baechle
Date: Fri Jun 14 2013 - 07:42:26 EST

On Fri, Jun 07, 2013 at 04:03:05PM -0700, David Daney wrote:
> Date: Fri, 7 Jun 2013 16:03:05 -0700
> From: David Daney <ddaney.cavm@xxxxxxxxx>
> To: linux-mips@xxxxxxxxxxxxxx, ralf@xxxxxxxxxxxxxx, kvm@xxxxxxxxxxxxxxx,
> Sanjay Lal <sanjayl@xxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx, David Daney <ddaney@xxxxxxxxxxxxxxxxxx>
> Subject: [PATCH 01/31] MIPS: Move allocate_kscratch to cpu-probe.c and make
> it public.
> From: David Daney <ddaney@xxxxxxxxxxxxxxxxxx>

I'd just like to add a note about compatibility. Code optimized for
LL/SC-less CPUs has made use of the fact that exception handlers will
clobber k0/k1 to a non-zero value. On a MIPS II or better CPU a branch
likely instruction could be used to atomically test k0/k1 and depending
on the test, execute a store instruction like:

.set noreorder
beqzl $k0, ok
sw $reg, offset($reg)
/* if we get here, our SC emulation has failed */
ok: ...

In particular Sony had elected to do this for the R5900 (after I explained
the concept to somebody and told it'd be a _bad_ idea for compatibility
reasons). Bad ideas are infectious so I'm sure others have used it, too.

I don't think this should stop your patch nor should we unless this turns
out to be an actual problem add any kludges to support such cowboy style
hacks. But I wanted to mention and document the issue; maybe this should
be mentioned in the log message of the next version of this patch.

Acked-by: Ralf Baechle <ralf@xxxxxxxxxxxxxx>

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