Re: [PATCH V2 4/8] perf: Capture branch privilege information

On 3/10/22 15:51, James Clark wrote:

On 09/03/2022 03:36, Anshuman Khandual wrote:

quoted

Platforms like arm64 could capture privilege level information for all the
branch records. Hence this adds a new element in the struct branch_entry to
record the privilege level information, which could be requested through a
new event.attr.branch_sample_type based flag PERF_SAMPLE_BRANCH_PRIV_SAVE.
This flag helps user choose whether privilege information is captured.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <redacted>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Gleixner <redacted>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-perf-users@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <redacted>
---
 include/uapi/linux/perf_event.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index d29280adc3c4..0e96e2017f68 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h

@@ -204,6 +204,8 @@ enum perf_branch_sample_type_shift {
 
 	PERF_SAMPLE_BRANCH_HW_INDEX_SHIFT	= 17, /* save low level index of raw branch records */
 
+	PERF_SAMPLE_BRANCH_PRIV_SAVE_SHIFT	= 18, /* save privillege mode */
+
 	PERF_SAMPLE_BRANCH_MAX_SHIFT		/* non-ABI */
 };
 

@@ -233,6 +235,8 @@ enum perf_branch_sample_type {
 
 	PERF_SAMPLE_BRANCH_HW_INDEX	= 1U << PERF_SAMPLE_BRANCH_HW_INDEX_SHIFT,
 
+	PERF_SAMPLE_BRANCH_PRIV_SAVE	= 1U << PERF_SAMPLE_BRANCH_PRIV_SAVE_SHIFT,
+
 	PERF_SAMPLE_BRANCH_MAX		= 1U << PERF_SAMPLE_BRANCH_MAX_SHIFT,
 };
 

@@ -271,6 +275,12 @@ enum {
 	PERF_BR_NEW_MAX,
 };
 
+enum {
+	PERF_BR_USER	= 0,
+	PERF_BR_KERNEL	= 1,
+	PERF_BR_HV	= 2,
+};

0 should be "PERF_BR_PRIV_UNKNOWN" so userspace knows if it was not enabled
otherwise it will look like all samples are PERF_BR_USER when actually
priv type recording was just disabled.

Makes sense.

I think it's not even always possible to go backwards from a sample to
work out what the event attributes were so this can be interpreted (taking
all of perf script and every corner case into account).

Starting at 0=UNKNOWN is consistent with the other fields and makes parsing
it a whole lot easier.

Fair enough, will start the enum with PERF_BR_PRIV_UNKNOWN and expand the
field 'perf_branch_entry.priv' to 3 bits instead, in order to accommodate
any future needs regarding other captured privilege levels.

James

quoted

+
 #define PERF_SAMPLE_BRANCH_PLM_ALL \
 	(PERF_SAMPLE_BRANCH_USER|\
 	 PERF_SAMPLE_BRANCH_KERNEL|\

@@ -1386,7 +1396,8 @@ struct perf_branch_entry {
 		cycles:16,  /* cycle count to last branch */
 		type:4,     /* branch type */
 		new_type:4, /* additional branch type */
-		reserved:36;
+		priv:2,     /* privilege level */
+		reserved:34;
 };
 
 union perf_sample_weight {

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel