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

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Date:   Tue, 12 Jan 2021 16:58:26 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Mike Leach <mike.leach@...aro.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        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

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.

For example, in the userspace, perf tool sets bit ETM_OPT_TS to enable
timestamp [1], this is same for ETMv3 and ETMv4.  In the kernel side,
the configuration is directly used ETMv3 (in coresight-etm3x-core.c),
but the configuration bits are converted for ETMv4 in the function
etm4_parse_event_config() [2].

So this is a historical issue, at the early period ETMv3 and ETMv4 can
be compatible with each other for configurations, but after evoluation,
some configs only belong to ETMv4 and cannot be applied on ETMv3
anymore, but we still use ETMv3.5 config bits as the interface between
kernel and userspace.

I'd like suggest we use a saperate patch set to refactor the
configuration bits, e.g. define the arbitrary configuration bits as
interface between user space and kernel, these bits are neutral for any
ETM version, in the kernel ETM drivers need to convert to its own
configuration formats.  For this patch, I want to keep current
change as it is, is this okay for you?

Thanks,
Leo

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/arch/arm/util/cs-etm.c?h=v5.11-rc3#n152
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/hwtracing/coresight/coresight-etm4x-core.c?h=v5.11-rc3#n422