Stephane Eranian <eran...@google.com> wrote: > On Wed, May 8, 2013 at 5:59 PM, Peter Zijlstra <pet...@infradead.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
