Re: [PATCH V3 13/14] perf: aux: Add flags for the buffer format

[Mike Leach]

On Wed, 27 Jan 2021 at 08:56, Anshuman Khandual
<anshuman.khandual@xxxxxxx> wrote:
>
> From: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>
> Allocate a byte for advertising the PMU specific format type
> of the given AUX record. A PMU could end up providing hardware
> trace data in multiple format in a single session.
>
> e.g, The format of hardware buffer produced by CoreSight ETM
> PMU depends on the type of the "sink" device used for collection
> for an event (Traditional TMC-ETR/Bs with formatting or
> TRBEs without any formatting).
>
>  # Boring story of why this is needed. Goto The_End_of_Story for skipping.
>
> CoreSight ETM trace allows instruction level tracing of Arm CPUs.
> The ETM generates the CPU excecution trace and pumps it into CoreSight
> AMBA Trace Bus and is collected by a different CoreSight component
> (traditionally CoreSight TMC-ETR /ETB/ETF), called "sink".
> Important to note that there is no guarantee that every CPU has
> a dedicated sink.  Thus multiple ETMs could pump the trace data
> into the same "sink" and thus they apply additional formatting
> of the trace data for the user to decode it properly and attribute
> the trace data to the corresponding ETM.
>
> However, with the introduction of Arm Trace buffer Extensions (TRBE),
> we now have a dedicated per-CPU architected sink for collecting the
> trace. Since the TRBE is always per-CPU, it doesn't apply any formatting
> of the trace. The support for this driver is under review [1].
>
> Now a system could have a per-cpu TRBE and one or more shared
> TMC-ETRs on the system. A user could choose a "specific" sink
> for a perf session (e.g, a TMC-ETR) or the driver could automatically
> select the nearest sink for a given ETM. It is possible that
> some ETMs could end up using TMC-ETR (e.g, if the TRBE is not
> usable on the CPU) while the others using TRBE in a single
> perf session. Thus we now have "formatted" trace collected
> from TMC-ETR and "unformatted" trace collected from TRBE.
> However, we don't get into a situation where a single event
> could end up using TMC-ETR & TRBE. i.e, any AUX buffer is
> guaranteed to be either RAW or FORMATTED, but not a mix
> of both.
>
> As for perf decoding, we need to know the type of the data
> in the individual AUX buffers, so that it can set up the
> "OpenCSD" (library for decoding CoreSight trace) decoder
> instance appropriately. Thus the perf.data file must conatin
> the hints for the tool to decode the data correctly.
>
> Since this is a runtime variable, and perf tool doesn't have
> a control on what sink gets used (in case of automatic sink
> selection), we need this information made available from
> the PMU driver for each AUX record.
>
>  # The_End_of_Story
>
> Cc: Peter Ziljstra <peterz@xxxxxxxxxxxxx>
> Cc: alexander.shishkin@xxxxxxxxxxxxxxx
> Cc: mingo@xxxxxxxxxx
> Cc: will@xxxxxxxxxx
> Cc: mark.rutland@xxxxxxx
> Cc: mike.leach@xxxxxxxxxx
> Cc: acme@xxxxxxxxxx
> Cc: jolsa@xxxxxxxxxx
> Cc: Mathieu Poirier <mathieu.poirer@xxxxxxxxxx>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>  include/uapi/linux/perf_event.h | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index b15e344..9a5ca45 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -1105,10 +1105,11 @@ enum perf_callchain_context {
>  /**
>   * PERF_RECORD_AUX::flags bits
>   */
> -#define PERF_AUX_FLAG_TRUNCATED                0x01    /* record was truncated to fit */
> -#define PERF_AUX_FLAG_OVERWRITE                0x02    /* snapshot from overwrite mode */
> -#define PERF_AUX_FLAG_PARTIAL          0x04    /* record contains gaps */
> -#define PERF_AUX_FLAG_COLLISION                0x08    /* sample collided with another */
> +#define PERF_AUX_FLAG_TRUNCATED                        0x01    /* record was truncated to fit */
> +#define PERF_AUX_FLAG_OVERWRITE                        0x02    /* snapshot from overwrite mode */
> +#define PERF_AUX_FLAG_PARTIAL                  0x04    /* record contains gaps */
> +#define PERF_AUX_FLAG_COLLISION                        0x08    /* sample collided with another */
> +#define PERF_AUX_FLAG_PMU_FORMAT_TYPE_MASK     0xff00  /* PMU specific trace format type */
>
>  #define PERF_FLAG_FD_NO_GROUP          (1UL << 0)
>  #define PERF_FLAG_FD_OUTPUT            (1UL << 1)
> --
> 2.7.4
>

Reviewed by: Mike Leach <mike.leach@xxxxxxxxxx>
-- 
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK