[PATCH 2/2] perf header: Additional note on AMD IBS for max_precise pmu cap

[Ravi Bangoria <ravi.bangoria@amd.com>]

From: Arnaldo Carvalho de Melo <acme@kernel.org>

x86 core pmu exposes supported maximum precision level via max_precise
pmu capability. Although, AMD core pmu does not support precise mode,
certain core pmu events with precise_ip > 0 are allowed and forwarded
to IBS OP pmu. Display a note about this in perf report header and
document the details in the perf-list man page.

Signed-off-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
---
 tools/perf/Documentation/perf-list.txt | 12 +++++++-----
 tools/perf/util/env.c                  | 18 ++++++++++++++++++
 tools/perf/util/env.h                  |  2 ++
 tools/perf/util/header.c               |  8 ++++++++
 4 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/tools/perf/Documentation/perf-list.txt b/tools/perf/Documentation/perf-list.txt
index d5f78e125efe..1b90575ee3c8 100644
--- a/tools/perf/Documentation/perf-list.txt
+++ b/tools/perf/Documentation/perf-list.txt
@@ -81,11 +81,13 @@ For Intel systems precise event sampling is implemented with PEBS
 which supports up to precise-level 2, and precise level 3 for
 some special cases
 
-On AMD systems it is implemented using IBS (up to precise-level 2).
-The precise modifier works with event types 0x76 (cpu-cycles, CPU
-clocks not halted) and 0xC1 (micro-ops retired). Both events map to
-IBS execution sampling (IBS op) with the IBS Op Counter Control bit
-(IbsOpCntCtl) set respectively (see the
+On AMD systems it is implemented using IBS OP (up to precise-level 2).
+Unlike Intel PEBS which provides levels of precision, AMD core pmu is
+inherently non-precise and IBS is inherently precise. (i.e. ibs_op//,
+ibs_op//p, ibs_op//pp and ibs_op//ppp are all same). The precise modifier
+works with event types 0x76 (cpu-cycles, CPU clocks not halted) and 0xC1
+(micro-ops retired). Both events map to IBS execution sampling (IBS op)
+with the IBS Op Counter Control bit (IbsOpCntCtl) set respectively (see the
 Core Complex (CCX) -> Processor x86 Core -> Instruction Based Sampling (IBS)
 section of the [AMD Processor Programming Reference (PPR)] relevant to the
 family, model and stepping of the processor being used).
diff --git a/tools/perf/util/env.c b/tools/perf/util/env.c
index 44140b7f596a..cbc18b22ace5 100644
--- a/tools/perf/util/env.c
+++ b/tools/perf/util/env.c
@@ -531,6 +531,24 @@ int perf_env__numa_node(struct perf_env *env, struct perf_cpu cpu)
 	return cpu.cpu >= 0 && cpu.cpu < env->nr_numa_map ? env->numa_map[cpu.cpu] : -1;
 }
 
+bool perf_env__has_pmu_mapping(struct perf_env *env, const char *pmu_name)
+{
+	char *pmu_mapping = env->pmu_mappings, *colon;
+
+	for (int i = 0; i < env->nr_pmu_mappings; ++i) {
+		if (strtoul(pmu_mapping, &colon, 0) == ULONG_MAX || *colon != ':')
+			goto out_error;
+
+		pmu_mapping = colon + 1;
+		if (strcmp(pmu_mapping, pmu_name) == 0)
+			return true;
+
+		pmu_mapping += strlen(pmu_mapping) + 1;
+	}
+out_error:
+	return false;
+}
+
 char *perf_env__find_pmu_cap(struct perf_env *env, const char *pmu_name,
 			     const char *cap)
 {
diff --git a/tools/perf/util/env.h b/tools/perf/util/env.h
index 4566c51f2fd9..56aea562c61b 100644
--- a/tools/perf/util/env.h
+++ b/tools/perf/util/env.h
@@ -174,4 +174,6 @@ struct btf_node *perf_env__find_btf(struct perf_env *env, __u32 btf_id);
 int perf_env__numa_node(struct perf_env *env, struct perf_cpu cpu);
 char *perf_env__find_pmu_cap(struct perf_env *env, const char *pmu_name,
 			     const char *cap);
+
+bool perf_env__has_pmu_mapping(struct perf_env *env, const char *pmu_name);
 #endif /* __PERF_ENV_H */
diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index e86b9439ffee..3cc288d14002 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -2145,6 +2145,14 @@ static void print_pmu_caps(struct feat_fd *ff, FILE *fp)
 		__print_pmu_caps(fp, pmu_caps->nr_caps, pmu_caps->caps,
 				 pmu_caps->pmu_name);
 	}
+
+	if (strcmp(perf_env__arch(&ff->ph->env), "x86") == 0 &&
+	    perf_env__has_pmu_mapping(&ff->ph->env, "ibs_op")) {
+		char *max_precise = perf_env__find_pmu_cap(&ff->ph->env, "cpu", "max_precise");
+
+		if (max_precise != NULL && atoi(max_precise) == 0)
+			fprintf(fp, "# AMD systems uses ibs_op// PMU for some precise events, e.g.: cycles:p, see the 'perf list' man page for further details.\n");
+	}
 }
 
 static void print_pmu_mappings(struct feat_fd *ff, FILE *fp)
-- 
2.41.0