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

[Ralf Baechle]

On Fri, Jun 14, 2013 at 01:41:18PM +0200, Ralf Baechle wrote:
> Date:   Fri, 14 Jun 2013 13:41:18 +0200
> From: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
> To: James Hogan <james.hogan@xxxxxxxxxx>
> Cc: linux-mips@xxxxxxxxxxxxxx, kvm@xxxxxxxxxxxxxxx, Sanjay Lal
>  <sanjayl@xxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, David Daney
>  <ddaney@xxxxxxxxxxxxxxxxxx>
> Subject: Re: [PATCH 01/31] MIPS: Move allocate_kscratch to cpu-probe.c and
>  make it public.
> Content-Type: text/plain; charset=us-ascii
> 
> 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>

Bleh.  I fatfingered mutt.  This of course was the reply intended for
"[PATCH 02/31] MIPS: Save and restore K0/K1 when CONFIG_KVM_MIPSVZ".

As for 1/31:

Acked-by: Ralf Baechle <ralf@xxxxxxxxxxxxxx>

  Ralf
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