perf/core: Add new branch sample type for HW index of raw branch records

The low level index is the index in the underlying hardware buffer of
the most recently captured taken branch which is always saved in
branch_entries[0]. It is very useful for reconstructing the call stack.
For example, in Intel LBR call stack mode, the depth of reconstructed
LBR call stack limits to the number of LBR registers. With the low level
index information, perf tool may stitch the stacks of two samples. The
reconstructed LBR call stack can break the HW limitation.

Add a new branch sample type to retrieve low level index of raw branch
records. The low level index is between -1 (unknown) and max depth which
can be retrieved in /sys/devices/cpu/caps/branches.

Only when the new branch sample type is set, the low level index
information is dumped into the PERF_SAMPLE_BRANCH_STACK output.
Perf tool should check the attr.branch_sample_type, and apply the
corresponding format for PERF_SAMPLE_BRANCH_STACK samples.
Otherwise, some user case may be broken. For example, users may parse a
perf.data, which include the new branch sample type, with an old version
perf tool (without the check). Users probably get incorrect information
without any warning.

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20200127165355.27495-2-kan.liang@linux.intel.com

1 files changed, 12 insertions, 0 deletions

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 547773f5894e..68e21e828893 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -93,14 +93,26 @@ struct perf_raw_record {
 /*
  * branch stack layout:
  *  nr: number of taken branches stored in entries[]
+ *  hw_idx: The low level index of raw branch records
+ *          for the most recent branch.
+ *          -1ULL means invalid/unknown.
  *
  * Note that nr can vary from sample to sample
  * branches (to, from) are stored from most recent
  * to least recent, i.e., entries[0] contains the most
  * recent branch.
+ * The entries[] is an abstraction of raw branch records,
+ * which may not be stored in age order in HW, e.g. Intel LBR.
+ * The hw_idx is to expose the low level index of raw
+ * branch record for the most recent branch aka entries[0].
+ * The hw_idx index is between -1 (unknown) and max depth,
+ * which can be retrieved in /sys/devices/cpu/caps/branches.
+ * For the architectures whose raw branch records are
+ * already stored in age order, the hw_idx should be 0.
  */
 struct perf_branch_stack {
 	__u64				nr;
+	__u64				hw_idx;
 	struct perf_branch_entry	entries[0];
 };