linux-kernel - [PATCH] perf/x86/intel/lbr: fix branch type encoding

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Message-Id: <20220810210656.2799243-1-eranian@google.com>
Date:   Wed, 10 Aug 2022 14:06:56 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, kan.liang@...el.com, ak@...ux.intel.com,
        acme@...hat.com, namhyung@...nel.org, irogers@...gle.com
Subject: [PATCH] perf/x86/intel/lbr: fix branch type encoding

With architected LBR, the procesosr can record the type of each sampled taken
branch. The type is encoded in 4-bit field in the LBR_INFO MSR of each entry.

The branch type must then extracted and saved in the perf_branch_entry in the
perf_events sampling buffer. With the current code, the raw Intel encoding of
the branch is exported to user tools. Yet tools, such as perf, expected the
branch type to be encoded using perf_events branch type enum
(see tools/perf/util/branch.c). As a result of the discrepancy, the output of
perf report -D shows bogus branch types.

Fix the problem by converting the Intel raw encoding into the perf_events
branch type enum values. With that in place and with no changes to the tools,
the branch types are now reported properly.

Signed-off-by: Stephane Eranian <eranian@...gle.com>
---
 arch/x86/events/intel/lbr.c | 35 ++++++++++++++++++++++++++++++++---
 1 file changed, 32 insertions(+), 3 deletions(-)

diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
index 4f70fb6c2c1e..ef63d4d46b50 100644
--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -894,9 +894,23 @@ static DEFINE_STATIC_KEY_FALSE(x86_lbr_mispred);
 static DEFINE_STATIC_KEY_FALSE(x86_lbr_cycles);
 static DEFINE_STATIC_KEY_FALSE(x86_lbr_type);
 
-static __always_inline int get_lbr_br_type(u64 info)
+/*
+ * Array index encodes IA32_LBR_x_INFO Branch Type Encodings
+ * as per Intel SDM Vol3b Branch Types section
+ */
+static const int arch_lbr_type_map[]={
+	[0] = PERF_BR_COND,
+	[1] = PERF_BR_IND,
+	[2] = PERF_BR_UNCOND,
+	[3] = PERF_BR_IND_CALL,
+	[4] = PERF_BR_CALL,
+	[5] = PERF_BR_RET,
+};
+#define ARCH_LBR_TYPE_COUNT ARRAY_SIZE(arch_lbr_type_map)
+
+static __always_inline u16 get_lbr_br_type(u64 info)
 {
-	int type = 0;
+	u16 type = 0;
 
 	if (static_branch_likely(&x86_lbr_type))
 		type = (info & LBR_INFO_BR_TYPE) >> LBR_INFO_BR_TYPE_OFFSET;
@@ -904,6 +918,21 @@ static __always_inline int get_lbr_br_type(u64 info)
 	return type;
 }
 
+/*
+ * The kernel cannot expose raw Intel branch type encodings because they are
+ * not generic. Instead, the function below  maps the encoding to the
+ * perf_events user visible branch types.
+ */
+static __always_inline int get_lbr_br_type_mapping(u64 info)
+{
+	if (static_branch_likely(&x86_lbr_type)) {
+		u16 raw_type = get_lbr_br_type(info);
+		if (raw_type < ARCH_LBR_TYPE_COUNT)
+			return arch_lbr_type_map[raw_type];
+	}
+	return PERF_BR_UNKNOWN;
+}
+
 static __always_inline bool get_lbr_mispred(u64 info)
 {
 	bool mispred = 0;
@@ -957,7 +986,7 @@ static void intel_pmu_store_lbr(struct cpu_hw_events *cpuc,
 		e->in_tx	= !!(info & LBR_INFO_IN_TX);
 		e->abort	= !!(info & LBR_INFO_ABORT);
 		e->cycles	= get_lbr_cycles(info);
-		e->type		= get_lbr_br_type(info);
+		e->type		= get_lbr_br_type_mapping(info);
 	}
 
 	cpuc->lbr_stack.nr = i;
-- 
2.37.1.559.g78731f0fdb-goog