Re: IV.5 - Intel Last Branch Record (LBR)

Ingo Molnar
2009-06-22T12:01:58Z

> 5/ Intel Last Branch Record (LBR)
>
> Intel processors since Netburst have a cyclic buffer hosted in
> registers which can record taken branches. Each taken branch is
> stored into a pair of LBR registers (source, destination). Up
> until Nehalem, there was not filtering capabilities for LBR. LBR
> is not an architected PMU feature.
>
> There is no counter associated with LBR. Nehalem has a LBR_SELECT
> MSR. However there are some constraints on it given it is shared
> by threads.
>
> LBR is only useful when sampling and therefore must be combined
> with a counter. LBR must also be configured to freeze on PMU
> interrupt.
>
> How is LBR going to be supported?

If there's interest then one sane way to support it would be to
expose it as a new sampling format (PERF_SAMPLE_*).

Regarding the constraints - if we choose to expose the branch-type
filtering capabilities of Nehalem, then that puts a constraint on
counter scheduling: two counters with conflicting constraints should
not be scheduled at once, but should be time-shared via the usual
mechanism.

The typical use-case would be to have no or compatible LBR filter
attributes between counters though - so having the conflicts is not
an issue as long as it works according to the usual model.