Re: [PATCH 2/2] PM / arch: x86: MSR_IA32_ENERGY_PERF_BIAS sysfs interface

[Borislav Petkov <bp@alien8.de>]

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?

> +	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? :)

> +		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;
> +}

-- 
Regards/Gruss,
    Boris.

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