linux-kernel - Re: [PATCHv3] perf kvm: add kvm-stat for arm64

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Date:   Thu, 17 Sep 2020 21:08:54 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Marc Zyngier <maz@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        John Garry <john.garry@...wei.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv3] perf kvm: add kvm-stat for arm64

Hi Sergey,

On Thu, Sep 17, 2020 at 08:00:28PM +0900, Sergey Senozhatsky wrote:
> On (20/09/17 11:21), Marc Zyngier wrote:
> > > > On Arm64, ftrace tracepoint "kvm_entry" doesn't contain the field "id"
> > > > or field "vcpu_id", thus it always reads out the "id" is 0 and it is
> 
> Right, Leo, I saw that. I put "id", because there is ... well, there is
> nothing we can put there. The trace points on arm64, unlike on s390 or x86,
> don't contain info which can let us map trace-event to vcpu_id (even
> (void *)vcpu would have been rather helpful).
> 
> > > > Essentially, this issue is caused by different archs using different
> > > > format for ftrace event "kvm_entry", on x86 it contains feild
> > > > "vcpu_id" but arm64 only just records "vcpu_pc".
> 
> Exactly. I wish trace-points were less of pain-points.
> 
> So 'perf kvm stat' on arm64 works, but it's not as feature rich as on other
> platforms; at the same it's better than nothing, I suppose.

Agree and I don't want to introduce complexity at here.  So it's fine
for me to merge your patch v4 firstly and later use a new patch set
to support "vcpu_id".

As you might have seen the discussion with Marc for new trace events
with "vcpu_id", another way is to add new trace events with "vcpu_id"
and then enable "perf kvm stat" based on new trace events.

Either way is okay for me; except maintainers have different thinking
for this.

Thanks,
Leo