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Re: [PATCH 01/31] MIPS: Move allocate_kscratch to cpu-probe.c and make i

To: James Hogan <james.hogan@imgtec.com>
Subject: Re: [PATCH 01/31] MIPS: Move allocate_kscratch to cpu-probe.c and make it public.
From: Ralf Baechle <ralf@linux-mips.org>
Date: Fri, 14 Jun 2013 13:41:18 +0200
Cc: linux-mips@linux-mips.org, kvm@vger.kernel.org, Sanjay Lal <sanjayl@kymasys.com>, linux-kernel@vger.kernel.org, David Daney <ddaney@caviumnetworks.com>
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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@gmail.com>
> To: linux-mips@linux-mips.org, ralf@linux-mips.org, kvm@vger.kernel.org,
>  Sanjay Lal <sanjayl@kymasys.com>
> Cc: linux-kernel@vger.kernel.org, David Daney <ddaney@caviumnetworks.com>
> Subject: [PATCH 01/31] MIPS: Move allocate_kscratch to cpu-probe.c and make
>  it public.
> 
> From: David Daney <ddaney@caviumnetworks.com>

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@linux-mips.org>

  Ralf

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