linux-kernel - Invalid perf_branch_entry.to entries question

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Date:	Tue, 07 May 2013 11:35:28 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, eranian@...gle.com
cc:	Linux PPC dev <linuxppc-dev@...abs.org>,
	linux-kernel@...r.kernel.org, michael@...erman.id.au,
	Anshuman Khandual <anshuman.khandual@...ibm.com>
Subject: Invalid perf_branch_entry.to entries question

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.  We
initially thought of just making this 0, but it's feasible that this
could be a valid address to branch to. 

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.

Mikey

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