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SubjectRe: [PATCH 2/2] PM / arch: x86: MSR_IA32_ENERGY_PERF_BIAS sysfs interface
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;
> > +}
>
> --

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