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From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Borislav Petkov <bp@alien8.de>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>, x86 <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Len Brown <len.brown@intel.com>,
	Linux PM <linux-pm@vger.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Laura Abbott <labbott@fedoraproject.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Simon Schricker <sschricker@suse.de>,
	Hannes Reinecke <hare@suse.de>
Subject: Re: [PATCH 2/2] PM / arch: x86: MSR_IA32_ENERGY_PERF_BIAS sysfs interface
Date: Mon, 25 Mar 2019 11:01:08 +0100	[thread overview]
Message-ID: <CAJZ5v0iML0dvzLszSnZjBPKc=2fWp4aUxEjAYWjJnz41Kz8w1g@mail.gmail.com> (raw)
In-Reply-To: <20190322144629.GC12472@zn.tnic>

On Fri, Mar 22, 2019 at 3:46 PM Borislav Petkov <bp@alien8.de> wrote:
>
> First of all, thanks a lot for doing that!
>
> This is a good example for how we should convert all the /dev/msr
> accessing tools.
>
> Nitpicks below.
>
> On Thu, Mar 21, 2019 at 11:20:17PM +0100, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> >
> > The Performance and Energy Bias Hint (EPB) is expected to be set by
> > user space through the generic MSR interface, but that interface is
> > not particularly nice and there are security concerns regarding it,
> > so it is not always available.
> >
> > For this reason, add a sysfs interface for reading and updating the
> > EPB, in the form of a new attribute, energy_perf_bias, located
> > under /sys/devices/system/cpu/cpu#/power/ for online CPUs that
> > support the EPB feature.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > ---
> >  Documentation/ABI/testing/sysfs-devices-system-cpu |   18 ++++
> >  Documentation/admin-guide/pm/intel_epb.rst         |   27 ++++++
> >  arch/x86/kernel/cpu/intel_epb.c                    |   93 ++++++++++++++++++++-
> >  3 files changed, 134 insertions(+), 4 deletions(-)
>
> ...
>
> > +static ssize_t energy_perf_bias_show(struct device *dev,
> > +                                  struct device_attribute *attr,
> > +                                  char *buf)
> > +{
> > +     unsigned int cpu = dev->id;
> > +     u64 epb;
> > +     int ret;
> > +
> > +     ret = rdmsrl_on_cpu(cpu, MSR_IA32_ENERGY_PERF_BIAS, &epb);
>
> That's an IPI and an MSR read each time. You could dump saved_epb
> instead, no?

No, because the MSR can change in ways beyond control of this code sometimes.

Generally, saved_epb only is the right value at the CPU online time.

> > +     if (ret < 0)
> > +             return ret;
> > +
> > +     return sprintf(buf, "%llu\n", epb);
> > +}
> > +
> > +static ssize_t energy_perf_bias_store(struct device *dev,
> > +                                   struct device_attribute *attr,
> > +                                   const char *buf, size_t count)
> > +{
> > +     unsigned int cpu = dev->id;
> > +     u64 epb, val;
> > +     int ret;
> > +
> > +     ret = __sysfs_match_string(energy_perf_strings,
> > +                                ARRAY_SIZE(energy_perf_strings), buf);
> > +     if (ret >= 0)
> > +             val = energ_perf_values[ret];
> > +     else if (kstrtou64(buf, 0, &val) || val > MAX_EPB)
>
> Range is 0 - 15 but u64? Maybe make it an u8? :)

At the cost of an extra conversion below, right?

> > +             return -EINVAL;
> > +
> > +     ret = rdmsrl_on_cpu(cpu, MSR_IA32_ENERGY_PERF_BIAS, &epb);
> > +     if (ret < 0)
> > +             return ret;
> > +
> > +     ret = wrmsrl_on_cpu(cpu, MSR_IA32_ENERGY_PERF_BIAS,
> > +                         (epb & ~EPB_MASK) | val);
> > +     if (ret < 0)
> > +             return ret;
> > +
> > +     return count;
> > +}
>
> --

  reply	other threads:[~2019-03-25 10:01 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-21 22:12 [PATCH 0/2] PM / arch: x86: MSR_IA32_ENERGY_PERF_BIAS handling fixes and sysfs i/f Rafael J. Wysocki
2019-03-21 22:18 ` [PATCH 1/2] PM / arch: x86: Rework the MSR_IA32_ENERGY_PERF_BIAS handling Rafael J. Wysocki
2019-03-22  9:03   ` Hannes Reinecke
2019-03-22 14:28   ` Borislav Petkov
2019-03-22 14:31     ` Thomas Gleixner
2019-03-22 14:35       ` Borislav Petkov
2019-03-22 16:12         ` Thomas Gleixner
2019-03-22 16:52           ` Joe Perches
2019-03-25 10:06     ` Rafael J. Wysocki
2019-03-22 16:27   ` Thomas Renninger
2019-03-22 16:43     ` Borislav Petkov
2019-03-25 11:31   ` Borislav Petkov
2019-03-21 22:20 ` [PATCH 2/2] PM / arch: x86: MSR_IA32_ENERGY_PERF_BIAS sysfs interface Rafael J. Wysocki
2019-03-22  9:03   ` Hannes Reinecke
2019-03-22 14:46   ` Borislav Petkov
2019-03-25 10:01     ` Rafael J. Wysocki [this message]
2019-03-22 15:00   ` Peter Zijlstra
2019-03-25  9:56     ` Rafael J. Wysocki
2019-03-25 11:32   ` Borislav Petkov
2019-05-09 10:23   ` Ido Schimmel
2019-05-09 17:18     ` Rafael J. Wysocki
2019-05-09 17:43       ` Ido Schimmel
2019-05-09 21:28         ` [PATCH] x86: intel_epb: Take CONFIG_PM into account Rafael J. Wysocki
2019-05-10  6:01           ` Ingo Molnar
2019-05-27 10:56         ` [PATCH] x86: intel_epb: Do not build when CONFIG_PM is unset Rafael J. Wysocki
2019-05-30  7:47           ` Ingo Molnar

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