linux-kernel - Re: [PATCH v1 1/7] coresight: etm-perf: Add support for PID tracing for kernel at EL2

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Message-ID: <aa4ffdd3-120f-4970-e1fc-279419c85f1c@arm.com>
Date:   Tue, 12 Jan 2021 11:03:02 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Leo Yan <leo.yan@...aro.org>, Mike Leach <mike.leach@...aro.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Daniel Kiss <Daniel.Kiss@....com>,
        Denis Nikitin <denik@...omium.org>,
        Coresight ML <coresight@...ts.linaro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Grant <al.grant@....com>
Subject: Re: [PATCH v1 1/7] coresight: etm-perf: Add support for PID tracing
 for kernel at EL2

On 1/12/21 8:58 AM, Leo Yan wrote:
> Hi Mike,
> 
> On Mon, Jan 11, 2021 at 04:22:39PM +0000, Mike Leach wrote:
> 
> [...]
> 
>>> diff --git a/include/linux/coresight-pmu.h b/include/linux/coresight-pmu.h
>>> index b0e35eec6499..927c6285ce5d 100644
>>> --- a/include/linux/coresight-pmu.h
>>> +++ b/include/linux/coresight-pmu.h
>>> @@ -11,16 +11,19 @@
>>>   #define CORESIGHT_ETM_PMU_SEED  0x10
>>>
>>>   /* ETMv3.5/PTM's ETMCR config bit */
>>> -#define ETM_OPT_CYCACC  12
>>> -#define ETM_OPT_CTXTID 14
>>> -#define ETM_OPT_TS      28
>>> -#define ETM_OPT_RETSTK 29
>>> +#define ETM_OPT_CYCACC         12
>>> +#define ETM_OPT_CTXTID         14
>>> +#define ETM_OPT_CTXTID_IN_VMID 15
>>
>> Minor issue here - ETMv3.x / PTM cannot trace CXTID in VMID so this
>> may better be named ETM4_OPT_CTXTID_IN_VMID, rather than be grouped
>> with the ETM3.5 options?
> 
> I looked into this suggestion but found it's complex than I assumed.
> This config bits are not only used for ETMv3.x / PTM, it's also used
> as an configuration interface between user space in Perf and kernel
> drivers.
> 

Exactly. I believe this problematic. We are stuckwith using the ETM3.x/PTM
config bits for the CS_ETM pmu, which is a bit wierd and the allocation of
the config bits are sparse. The problem with changing them now, will break
older perf tools decoding a perf.data from a newer kernel. I believe we are
stuck with this.

I would recommend simply updating the comment to reflect that, thats the
generic CS PMU ABI for configuration which was initially based on ETM3.x.

Suzuki