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Date: Tue, 4 Oct 2016 09:06:18 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Jiri Olsa <jolsa@redhat.com>, Michael Neuling <mikey@neuling.org>,
        Paul Mackerras <paulus@samba.org>, Jiri Olsa <jolsa@kernel.org>,
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        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jan Stancek <jstancek@redhat.com>, Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH] perf powerpc: Don't call perf_event_disable from atomic
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On Tue, Oct 04, 2016 at 03:29:33PM +1100, Michael Ellerman wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> > So it would be good to also explain why PPC needs this in the first
> > place.
> 
> Unfortunately I don't really know the code, and the original author is AWOL.
> 
> But AFAICS perf_event_disable() is only called here:
> 
> 	if (!stepped) {
> 		WARN(1, "Unable to handle hardware breakpoint. Breakpoint at "
> 			"0x%lx will be disabled.", info->address);
> 		perf_event_disable(bp);
> 		goto out;
> 	}
> 
> Which is where we cope with the possibility that we couldn't emulate the
> instruction that hit the breakpoint. Seems that is not an issue on x86,
> or it's handled elsewhere?

I don't think x86 ever needs to emulate things on hw breakpoint
(although I could be mistaken), but I would expect ARM to maybe need
so, and I couldn't find a disable there either.

Will?

> We should fix emulate_step() if it failed to emulate something it
> should have, but there will always be the possibility that it fails.
> 
> Instead of calling perf_event_disable() we could just add a flag to
> arch_hw_breakpoint that says we hit an error on the event, and block
> reinstalling it in arch_install_hw_breakpoint().

Possible..