linux-kernel - Re: [PATCH v2 5/6] drivers/perf: Add support for ARMv8.2 Statistical Profiling Extension

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Message-Id: <20170407115241.841f6d259cfa4a1165d106c5@arm.com>
Date:   Fri, 7 Apr 2017 11:52:41 +0100
From:   Kim Phillips <kim.phillips@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Will Deacon <will.deacon@....com>,
        <linux-arm-kernel@...ts.infradead.org>, <marc.zyngier@....com>,
        <alex.bennee@...aro.org>, <christoffer.dall@...aro.org>,
        <tglx@...utronix.de>, <peterz@...radead.org>,
        <alexander.shishkin@...ux.intel.com>, <robh@...nel.org>,
        <suzuki.poulose@....com>, <pawel.moll@....com>,
        <mathieu.poirier@...aro.org>, <mingo@...hat.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/6] drivers/perf: Add support for ARMv8.2
 Statistical Profiling Extension

On Thu, 6 Apr 2017 19:45:04 +0100
Mark Rutland <mark.rutland@....com> wrote:

> On Thu, Apr 06, 2017 at 07:33:07PM +0100, Kim Phillips wrote:
> > On Thu, 6 Apr 2017 17:18:15 +0100
> > Will Deacon <will.deacon@....com> wrote:
> > 
> > Hi Will,
> > 
> > > +/* Perf callbacks */
> > > +static int arm_spe_pmu_event_init(struct perf_event *event)
> > > +{
> > > +	u64 reg;
> > > +	struct perf_event_attr *attr = &event->attr;
> > > +	struct arm_spe_pmu *spe_pmu = to_spe_pmu(event->pmu);
> > > +
> > > +	/* This is, of course, deeply driver-specific */
> > > +	if (attr->type != event->pmu->type)
> > > +		return -ENOENT;
> > > +
> > > +	if (event->cpu >= 0 &&
> > > +	    !cpumask_test_cpu(event->cpu, &spe_pmu->supported_cpus))
> > > +		return -ENOENT;
> > > +
> > > +	if (arm_spe_event_to_pmsevfr(event) & PMSEVFR_EL1_RES0)
> > > +		return -EOPNOTSUPP;
> > > +
> > > +	if (event->hw.sample_period < spe_pmu->min_period ||
> > > +	    event->hw.sample_period & PMSIRR_EL1_IVAL_MASK)
> > > +		return -EOPNOTSUPP;
> > > +
> > > +	if (attr->exclude_idle)
> > > +		return -EOPNOTSUPP;
> > > +
> > > +	/*
> > > +	 * Feedback-directed frequency throttling doesn't work when we
> > > +	 * have a buffer of samples. We'd need to manually count the
> > > +	 * samples in the buffer when it fills up and adjust the event
> > > +	 * count to reflect that. Instead, force the user to specify a
> > > +	 * sample period instead.
> > > +	 */
> > > +	if (attr->freq)
> > > +		return -EINVAL;
> > > +
> > > +	if (is_kernel_in_hyp_mode()) {
> > > +		if (attr->exclude_kernel != attr->exclude_hv)
> > > +			return -EOPNOTSUPP;
> > > +	} else if (!attr->exclude_hv) {
> > > +		return -EOPNOTSUPP;
> > > +	}
> > > +
> > > +	reg = arm_spe_event_to_pmsfcr(event);
> > > +	if ((reg & BIT(PMSFCR_EL1_FE_SHIFT)) &&
> > > +	    !(spe_pmu->features & SPE_PMU_FEAT_FILT_EVT))
> > > +		return -EOPNOTSUPP;
> > > +
> > > +	if ((reg & BIT(PMSFCR_EL1_FT_SHIFT)) &&
> > > +	    !(spe_pmu->features & SPE_PMU_FEAT_FILT_TYP))
> > > +		return -EOPNOTSUPP;
> > > +
> > > +	if ((reg & BIT(PMSFCR_EL1_FL_SHIFT)) &&
> > > +	    !(spe_pmu->features & SPE_PMU_FEAT_FILT_LAT))
> > > +		return -EOPNOTSUPP;
> > > +
> > > +	return 0;
> > > +}
> > 
> > Can you please explain why we're not emitting messages to dmesg here?:
> > 
> > https://patchwork.kernel.org/patch/9545979/
> 
> The above cases are not (system) errors, and using dev_err (even
> ratelimited) is certainly not appropriate. These are pr_debug() at best.

They are driver errors:  the input parameters to the driver were bad,
and it failed to execute.  pr_debug - to me at least - should indicate
progress during normal operation.

> The dmesg is not always the appropriate place to dump such information,
> even if it happens to be convenient. As part of usual operation, dmesg
> should be very quiet, and we don't log messages elsewhere where the user
> passes some parameter the kernel does not like.
> 
> These messages are really only useful to those developing tools (such as
> yourself).

We had a customer come back with a simple usage failure because they
were running a kernel with a different VHE configuration, blindly
failing the hv exclusion check above.  They had to manually modify the
driver to find the cause.  So it affects all users, not just me.
Unless you're implying the above code be duplicated in the perf tool
somehow?

> There are some cases where they're actively harmful (e.g.
> when fuzzing).

I'd expect fuzzer users to be more amenable to manually modifying the
driver rather than regular users of the driver.

> Adding a single patch doesn't seem that difficult.

Can we not address the issue in this original patch that causes it?

> Maybe we could add a pr_debug() or two.

If the diff I sent had too many, by all means, let's discuss in the
form of a review.

Kim