linux-kernel - Re: [RFC 1/2] powerpc/32: Unset MSR RI in exception epilogs

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Date:   Wed, 14 Dec 2016 09:40:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     christophe leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC 1/2] powerpc/32: Unset MSR RI in exception epilogs

On Tue, Dec 13, 2016 at 04:54:30PM -0600, Segher Boessenkool wrote:
> On Tue, Dec 13, 2016 at 09:39:55PM +0100, christophe leroy wrote:
> > Le 13/12/2016 à 20:15, Segher Boessenkool a écrit :
> > >On Tue, Dec 13, 2016 at 07:19:41PM +0100, Christophe Leroy wrote:
> > >>At exception prologs, once SRR0 and SRR1 have been saved, MSR RI is
> > >>set to mark the interrupt as recoverable.
> > >>
> > >>MSR RI has to be unset before writing into SRR0 and SRR1 at exception
> > >>epilogs.
> > >
> > >Why?  What goes wrong without this?  Etc.
> > 
> > The following patch implements perf instruction counting using the 8xx 
> > debug counters. When the counter reaches 0, it fires a debug exception.
> > If that exception happens between the setting of srr0/srr1 and the rfi, 
> > values set to srr0/srr1 are lost and we end up with an Oops.
> > 
> > To avoid that, MSR RI has to be unset. That way, because the debug 
> > counters mode is set to masked mode in register LCTRL2, no debug 
> > interrupt will happen during that critical phase.
> 
> Okay, so why then do you do an expensive sequence on all other processors?
> 

Does ppc32 support runtime code patching? If so, you could perhaps
utilize that to only inflict the painful code sequence when perf is
enabled.