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> Ralf
