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, 3 insertions, 0 deletions

diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
index 534c76606049..7639e2097101 100644
--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -585,6 +585,7 @@ static void intel_pmu_lbr_read_32(struct cpu_hw_events *cpuc)
 		cpuc->lbr_entries[i].reserved	= 0;
 	}
 	cpuc->lbr_stack.nr = i;
+	cpuc->lbr_stack.hw_idx = -1ULL;
 }
 
 /*
@@ -680,6 +681,7 @@ static void intel_pmu_lbr_read_64(struct cpu_hw_events *cpuc)
 		out++;
 	}
 	cpuc->lbr_stack.nr = out;
+	cpuc->lbr_stack.hw_idx = -1ULL;
 }
 
 void intel_pmu_lbr_read(void)
@@ -1120,6 +1122,7 @@ void intel_pmu_store_pebs_lbrs(struct pebs_lbr *lbr)
 	int i;
 
 	cpuc->lbr_stack.nr = x86_pmu.lbr_nr;
+	cpuc->lbr_stack.hw_idx = -1ULL;
 	for (i = 0; i < x86_pmu.lbr_nr; i++) {
 		u64 info = lbr->lbr[i].info;
 		struct perf_branch_entry *e = &cpuc->lbr_entries[i];