[PATCH 1/2] perf/x86/lbr: Enable the branch type for the Arch LBR by default

* [PATCH 1/2] perf/x86/lbr: Enable the branch type for the Arch LBR by default
@ 2022-08-16 12:56 kan.liang
  2022-08-16 12:56 ` [PATCH 2/2] perf docs: Update the documentation for the save_type filter kan.liang
  2022-08-22  9:00 ` [tip: perf/urgent] perf/x86/lbr: Enable the branch type for the Arch LBR by default tip-bot2 for Kan Liang
  0 siblings, 2 replies; 4+ messages in thread
From: kan.liang @ 2022-08-16 12:56 UTC (permalink / raw)
  To: peterz, acme, linux-kernel
  Cc: eranian, ak, namhyung, irogers, Kan Liang, stable

From: Kan Liang <kan.liang@linux.intel.com>

On the platform with Arch LBR, the HW raw branch type encoding may leak
to the perf tool when the SAVE_TYPE option is not set.

In the intel_pmu_store_lbr(), the HW raw branch type is stored in
lbr_entries[].type. If the SAVE_TYPE option is set, the
lbr_entries[].type will be converted into the generic PERF_BR_* type
in the intel_pmu_lbr_filter() and exposed to the user tools.
But if the SAVE_TYPE option is NOT set by the user, the current perf
kernel doesn't clear the field. The HW raw branch type leaks.

There are two solutions to fix the issue for the Arch LBR.
One is to clear the field if the SAVE_TYPE option is NOT set.
The other solution is to unconditionally convert the branch type and
expose the generic type to the user tools.

The latter is implemented here, because
- The branch type is valuable information. I don't see a case where
  you would not benefit from the branch type. (Stephane Eranian)
- Not having the branch type DOES NOT save any space in the
  branch record (Stephane Eranian)
- The Arch LBR HW can retrieve the common branch types from the
  LBR_INFO. It doesn't require the high overhead SW disassemble.

Fixes: 47125db27e47 ("perf/x86/intel/lbr: Support Architectural LBR")
Reported-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Cc: stable@vger.kernel.org
---
 arch/x86/events/intel/lbr.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
index a4a36d482b21..247d36825474 100644
--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -1149,6 +1149,14 @@ static int intel_pmu_setup_hw_lbr_filter(struct perf_event *event)
 
 	if (static_cpu_has(X86_FEATURE_ARCH_LBR)) {
 		reg->config = mask;
+
+		/*
+		 * The Arch LBR HW can retrieve the common branch types
+		 * from the LBR_INFO. It doesn't require the high overhead
+		 * SW disassemble.
+		 * Enable the branch type by default for the Arch LBR.
+		 */
+		reg->reg |= X86_BR_TYPE_SAVE;
 		return 0;
 	}
 
-- 
2.35.1


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