Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752501Ab3FNLmX (ORCPT ); Fri, 14 Jun 2013 07:42:23 -0400 Received: from eddie.linux-mips.org ([78.24.191.182]:37581 "EHLO cvs.linux-mips.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752370Ab3FNLmV (ORCPT ); Fri, 14 Jun 2013 07:42:21 -0400 Date: Fri, 14 Jun 2013 13:41:18 +0200 From: Ralf Baechle To: James Hogan Cc: linux-mips@linux-mips.org, kvm@vger.kernel.org, Sanjay Lal , linux-kernel@vger.kernel.org, David Daney Subject: Re: [PATCH 01/31] MIPS: Move allocate_kscratch to cpu-probe.c and make it public. Message-ID: <20130614114118.GD15775@linux-mips.org> References: <1370646215-6543-1-git-send-email-ddaney.cavm@gmail.com> <1370646215-6543-2-git-send-email-ddaney.cavm@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1370646215-6543-2-git-send-email-ddaney.cavm@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1733 Lines: 40 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 > To: linux-mips@linux-mips.org, ralf@linux-mips.org, kvm@vger.kernel.org, > Sanjay Lal > Cc: linux-kernel@vger.kernel.org, David Daney > Subject: [PATCH 01/31] MIPS: Move allocate_kscratch to cpu-probe.c and make > it public. > > From: David Daney 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/