Invalid perf_branch_entry.to entries question

[Michael Neuling]

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