Re: [PATCH 2/3] perf: arm-spe: Fix SPE events with phys addresses

From: Leo Yan <leo.yan@linaro.org>
To: James Clark <james.clark@arm.com>
Cc: Timothy Hayes <timothy.hayes@arm.com>,
	linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
	acme@kernel.org, John Garry <john.garry@huawei.com>,
	Will Deacon <will@kernel.org>,
	Mathieu Poirier <mathieu.poirier@linaro.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@kernel.org>, Namhyung Kim <namhyung@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org,
	bpf@vger.kernel.org
Subject: Re: [PATCH 2/3] perf: arm-spe: Fix SPE events with phys addresses
Date: Tue, 26 Apr 2022 21:19:52 +0800	[thread overview]
Message-ID: <20220426131952.GA1923836@leoy-ThinkPad-X240s> (raw)
In-Reply-To: <322009d2-330c-22d4-4075-eca2042f64e1@arm.com>

On Mon, Apr 25, 2022 at 10:12:36AM +0100, James Clark wrote:

[...]

> >> diff --git a/tools/perf/util/arm-spe.c b/tools/perf/util/arm-spe.c
> >> index 151cc38a171c..1a80151baed9 100644
> >> --- a/tools/perf/util/arm-spe.c
> >> +++ b/tools/perf/util/arm-spe.c
> >> @@ -1033,7 +1033,8 @@ arm_spe_synth_events(struct arm_spe *spe, struct perf_session *session)
> >>  	memset(&attr, 0, sizeof(struct perf_event_attr));
> >>  	attr.size = sizeof(struct perf_event_attr);
> >>  	attr.type = PERF_TYPE_HARDWARE;
> >> -	attr.sample_type = evsel->core.attr.sample_type & PERF_SAMPLE_MASK;
> >> +	attr.sample_type = evsel->core.attr.sample_type &
> >> +				(PERF_SAMPLE_MASK | PERF_SAMPLE_PHYS_ADDR);
> > 
> > I verified this patch and I can confirm the physical address can be
> > dumped successfully.
> > 
> > I have a more general question, seems to me, we need to change the
> > macro PERF_SAMPLE_MASK in the file util/event.h as below, so
> > here doesn't need to 'or' the flag PERF_SAMPLE_PHYS_ADDR anymore.
> > 
> > @Arnaldo, @Jiri, could you confirm if this is the right way to move
> > forward?  I am not sure why PERF_SAMPLE_MASK doesn't contain the bit
> > PERF_SAMPLE_PHYS_ADDR in current code.
> 
> I think there is a reason that PERF_SAMPLE_MASK is a subset of all the
> bits. This comment below suggests it. Is it so the mask only includes fields
> that are 64bits? That makes the __evsel__sample_size() function a simple
> multiplication of a count of all the fields that are 64bits.

After reading code, seems the conclusion "a count of all the fields
that are 64bits" is not valid.  PERF_SAMPLE_MASK contains bits
PERF_SAMPLE_IP and PERF_SAMPLE_TID, the corresponding fields 'pid'
and 'tid' in the structure perf_sample are u32 type.

>   static int
>   perf_event__check_size(union perf_event *event, unsigned int sample_size)
>   {
> 	/*
> 	 * The evsel's sample_size is based on PERF_SAMPLE_MASK which includes
> 	 * up to PERF_SAMPLE_PERIOD.  After that overflow() must be used to
> 	 * check the format does not go past the end of the event.
> 	 */
> 	if (sample_size + sizeof(event->header) > event->header.size)
> 		return -EFAULT;

Okay, thanks for sharing the info, it does show that it's deliberately to
not contain all fields in PERF_SAMPLE_MASK.  If so, this patch is fine
for me:

Reviewed-by: Leo Yan <leo.yan@linaro.org>

> 	return 0;
>   }
> 
> Having said that, the mask was updated once to add PERF_SAMPLE_IDENTIFIER to
> it, so that comment is slightly out of date now.
> 
> 
> > 
> > diff --git a/tools/perf/util/event.h b/tools/perf/util/event.h
> > index cdd72e05fd28..c905ac32ebad 100644
> > --- a/tools/perf/util/event.h
> > +++ b/tools/perf/util/event.h
> > @@ -39,7 +39,7 @@ struct perf_event_attr;
> >          PERF_SAMPLE_TIME | PERF_SAMPLE_ADDR |          \
> >         PERF_SAMPLE_ID | PERF_SAMPLE_STREAM_ID |        \
> >          PERF_SAMPLE_CPU | PERF_SAMPLE_PERIOD |         \
> > -        PERF_SAMPLE_IDENTIFIER)
> > +        PERF_SAMPLE_IDENTIFIER | PERF_SAMPLE_PHYS_ADDR)
> > 
> > Thanks,
> > Leo
> > 
> >>  	attr.sample_type |= PERF_SAMPLE_IP | PERF_SAMPLE_TID |
> >>  			    PERF_SAMPLE_PERIOD | PERF_SAMPLE_DATA_SRC |
> >>  			    PERF_SAMPLE_WEIGHT | PERF_SAMPLE_ADDR;
> >> -- 
> >> 2.25.1
> >>