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
