linux-kernel - Re: Invalid perf_branch_entry.to entries question

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Message-ID: <15099.1368053151@ale.ozlabs.ibm.com>
Date:	Thu, 09 May 2013 08:45:51 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	Stephane Eranian <eranian@...gle.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux PPC dev <linuxppc-dev@...abs.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michael Ellerman <michael@...erman.id.au>,
	Anshuman Khandual <anshuman.khandual@...ibm.com>
Subject: Re: Invalid perf_branch_entry.to entries question

Stephane Eranian <eranian@...gle.com> wrote:

> On Wed, May 8, 2013 at 5:59 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Tue, May 07, 2013 at 11:35:28AM +1000, Michael Neuling wrote:
> >> Peter & Stephane,
> >>
> >> We are plumbing the POWER8 Branch History Rolling Buffer (BHRB) into
> >> struct perf_branch_entry.
> >>
> >> Sometimes on POWER8 we may not be able to fill out the "to" address.
> >
> > Just because I'm curious.. however does that happen? Surely the CPU knows where
> > next to fetch instructions?
> >
> >> We
> >> initially thought of just making this 0, but it's feasible that this
> >> could be a valid address to branch to.
> >
> > Right, while highly unlikely, x86 actually has some cases where 0 address is
> > valid *shudder*..
> >
> >> The other logical value to indicate an invalid entry would be all 1s
> >> which is not possible (on POWER at least).
> >>
> >> Do you guys have a preference as to what we should use as an invalid
> >> entry?  This would have some consequences for the userspace tool also.
> >>
> >> The alternative would be to add a flag alongside mispred/predicted to
> >> indicate the validity of the "to" address.
> >
> > Either would work with me I suppose.. Stephane do you have any preference?
> 
> But if the 'to' is bogus, why not just drop the sample?
> That happens on x86 if the HW captured branches which do not correspond to
> user filter settings (due to bug).

We can I guess but it seems useful to log the from address when
possible.  

Can we log it and userspace tools can ignore it if it's not useful?

Mikey
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