Re: Invalid perf_branch_entry.to entries question

On Wed, May 8, 2013 at 5:59 PM, Peter Zijlstra [off-list ref] wrote:

On Tue, May 07, 2013 at 11:35:28AM +1000, Michael Neuling wrote:

quoted

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?

quoted

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*..

quoted

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).