8.9. Branch Record Buffer

The Branch Record Buffer (BRB) allows recording of branch paths for profiling purposes.

TF-A supports FEAT_BRBE and FEAT_BRBEv1p1, the latter of which enables branch recording at EL3.

8.9.1. Branch Records

Taken branch instructions and exceptions can generate branch records - the BRB may be configured to filter which circumstances they are generated under. Each branch record consists of three registers:

BRBINF<n>_EL1: Stores information about a branch record
BRBSRC<n>_EL1: Stores the source address of a branch record
BRBTGT<n>_EL1: Stores the destination address of a branch record

Where n is 0 to at most 31 (IMPLEMENTATION DEFINED).

Note

For more information, consult Chapter D19.4 of the Arm ARM.

8.9.2. Using TF-A profiling

TF-A provides methods to enable branch recording at all exception levels, accessible from include/lib/extensions/brbe.h. Profiling can be carried out as follows:

brbe_start_recording();
// code to be profiled
brbe_stop_recording();

If FEAT_BRBEv1p1 is supported, then EL3 payloads may be profiled.

Since the BRB is of finite size, it is prone to overflowing; the resulting buffer flush then causes already-captured profiling information to be lost. TF-A will dump the contents of the BRB when it overflows. This relies on a PMU counter (by default, counter 0) triggering a synchronous exception. Setup of this counter is performed when recording starts, but relies on FEAT_EBEP being implemented for correct function.

Note

For more information on the PMU, consult Chapter D13 of the Arm ARM and the Performance Monitoring Unit section of this documentation.

References

Arm ARM