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Message-ID: <0d04d7a0-9186-ce89-fe51-616c2af37a8d@arm.com>
Date: Mon, 14 Mar 2022 12:17:29 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: James Clark <james.clark@....com>, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org
Cc: Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
linux-perf-users@...r.kernel.org, Jiri Olsa <jolsa@...hat.com>,
Namhyung Kim <namhyung@...nel.org>,
Rob Herring <rob.herring@....com>,
Suzuki K Poulose <suzuki.poulose@....com>
Subject: Re: [RFC V1 11/11] perf: Capture branch privilege information
On 1/26/22 22:57, James Clark wrote:
>
> On 24/01/2022 04:30, Anshuman Khandual wrote:
>> 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 flag PERF_SAMPLE_BRANCH_PRIV_SAVE. While
>> here, update the BRBE driver as required.
>>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
>> Cc: Jiri Olsa <jolsa@...hat.com>
>> Cc: Namhyung Kim <namhyung@...nel.org>
>> Cc: linux-arm-kernel@...ts.infradead.org
>> Cc: linux-perf-users@...r.kernel.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>> drivers/perf/arm_pmu_brbe.c | 28 ++++++++++++++++++++++++
>> include/linux/perf_event.h | 5 +++++
>> include/uapi/linux/perf_event.h | 13 ++++++++++-
>> tools/include/uapi/linux/perf_event.h | 13 ++++++++++-
>> tools/perf/Documentation/perf-record.txt | 1 +
>> tools/perf/util/parse-branch-options.c | 1 +
>> 6 files changed, 59 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/perf/arm_pmu_brbe.c b/drivers/perf/arm_pmu_brbe.c
>> index 7cd1208c6c58..d4cbea74c148 100644
>> --- a/drivers/perf/arm_pmu_brbe.c
>> +++ b/drivers/perf/arm_pmu_brbe.c
>> @@ -270,6 +270,25 @@ static int brbe_fetch_perf_type(u64 brbinf)
>> }
>> }
>>
>> +static int brbe_fetch_perf_priv(u64 brbinf)
>> +{
>> + int brbe_el = brbe_fetch_el(brbinf);
>> +
>> + switch (brbe_el) {
>> + case BRBINF_EL_EL0:
>> + return PERF_BR_USER;
>> + case BRBINF_EL_EL1:
>> + return PERF_BR_KERNEL;
>> + case BRBINF_EL_EL2:
>> + if (is_kernel_in_hyp_mode())
>> + return PERF_BR_KERNEL;
>> + return PERF_BR_HV;
>> + default:
>> + pr_warn("unknown branch privilege captured\n");
>> + return -1;
>> + }
>> +}
>> +
>> static void capture_brbe_flags(struct pmu_hw_events *cpuc, struct perf_event *event,
>> u64 brbinf, int idx)
>> {
>> @@ -302,6 +321,15 @@ static void capture_brbe_flags(struct pmu_hw_events *cpuc, struct perf_event *ev
>> cpuc->brbe_entries[idx].in_tx = brbinf & BRBINF_TX;
>> }
>> }
>> +
>> + if (branch_sample_priv(event)) {
>> + /*
>> + * All these information (i.e branch privilege level) are not
>> + * available for source only branch records.
>> + */
>> + if (type != BRBINF_VALID_SOURCE)
>> + cpuc->brbe_entries[idx].priv = brbe_fetch_perf_priv(brbinf);
>> + }
>> }
>>
>> /*
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index 916ce5102b33..8021b6a30d86 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -1688,4 +1688,9 @@ static inline bool branch_sample_hw_index(const struct perf_event *event)
>> {
>> return event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_HW_INDEX;
>> }
>> +
>> +static inline bool branch_sample_priv(const struct perf_event *event)
>> +{
>> + return event->attr.branch_sample_type & PERF_SAMPLE_BRANCH_PRIV_SAVE;
>> +}
>> #endif /* _LINUX_PERF_EVENT_H */
>> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
>> index 361fdc6b87a0..4d77710f7a4e 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,
>> };
>>
>> @@ -265,6 +269,12 @@ enum {
>> PERF_BR_MAX,
>> };
>>
>> +enum {
>> + PERF_BR_USER = 0,
>> + PERF_BR_KERNEL = 1,
>> + PERF_BR_HV = 2,
>> +};
>> +
> Can we have 0 as "UNKNOWN". It's going to be difficult to parse files when privilege information
> isn't saved and get accurate results without that. For example if it's not set then presumably
> the field would be 0 (PERF_BR_USER), but that doesn't mean the samples are user in that case.
>
> I know you might be able to go backwards and look at what arguments were passed to the kernel but
> it's not guaranteed that the kernel honored the request anyway. There are also other platforms
> to think about etc.
>
> If you look at the branch type definitions above they start at 0 (PERF_BR_UNKNOWN) which I think
> works out quite nicely in the userspace code.
This is being taken care in the new BRBE related perf ABI changes series (V3).
https://lore.kernel.org/all/20220314055857.125421-1-anshuman.khandual@arm.com/
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