linux-kernel - Re: [PATCH 1/2] PM / arch: x86: Rework the MSR_IA32_ENERGY_PERF

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Date:   Fri, 22 Mar 2019 15:28:28 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Len Brown <len.brown@...el.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Laura Abbott <labbott@...oraproject.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Simon Schricker <sschricker@...e.de>,
        Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH 1/2] PM / arch: x86: Rework the MSR_IA32_ENERGY_PERF_BIAS
 handling

On Thu, Mar 21, 2019 at 11:18:01PM +0100, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> The current handling of MSR_IA32_ENERGY_PERF_BIAS in the kernel is
> problematic, because it may cause changes made by user space to that
> MSR (with the help of the x86_energy_perf_policy tool, for example)

One more reason to control MSR accesses from userspace. I'm working on
a series to even completely forbid accesses to some MSRs over /dev/msr
so I think accessing MSR_IA32_ENERGY_PERF_BIAS solely over the new
interface in patch 2 would be much better.

So, you're carrying those and you'd like to have an ACK from me?

Btw, a couple of nitpicks below.

> Index: linux-pm/arch/x86/kernel/cpu/intel_epb.c
> ===================================================================
> --- /dev/null
> +++ linux-pm/arch/x86/kernel/cpu/intel_epb.c
> @@ -0,0 +1,131 @@
> +// SPDX-License-Identifier: GPL-2.0

...

> +static DEFINE_PER_CPU(u8, saved_epb);
> +
> +#define EPB_MASK	0x0fULL
> +#define EPB_SAVED	0x10ULL
> +
> +static int intel_epb_save(void)

I'd drop that "intel_epb_" prefix from those static functions, but your
call...

> +{
> +	u64 epb;
> +
> +	rdmsrl(MSR_IA32_ENERGY_PERF_BIAS, epb);
> +	/*
> +	 * Ensure that saved_epb will always be nonzero after this write even if
> +	 * the EPB value read from the MSR is 0.
> +	 */
> +	this_cpu_write(saved_epb, (epb & EPB_MASK) | EPB_SAVED);
> +
> +	return 0;
> +}

...

> Index: linux-pm/Documentation/admin-guide/pm/intel_epb.rst
> ===================================================================
> --- /dev/null
> +++ linux-pm/Documentation/admin-guide/pm/intel_epb.rst

WARNING: Missing or malformed SPDX-License-Identifier tag in line 1
#345: FILE: Documentation/admin-guide/pm/intel_epb.rst:1:
+======================================

> @@ -0,0 +1,6 @@
> +======================================
> +Intel Performance and Energy Bias Hint
> +======================================
> +
> +.. kernel-doc:: arch/x86/kernel/cpu/intel_epb.c
> +   :doc: overview
> Index: linux-pm/Documentation/admin-guide/pm/working-state.rst
> ===================================================================
> --- linux-pm.orig/Documentation/admin-guide/pm/working-state.rst
> +++ linux-pm/Documentation/admin-guide/pm/working-state.rst
> @@ -8,3 +8,4 @@ Working-State Power Management
>     cpuidle
>     cpufreq
>     intel_pstate
> +   intel_epb
> 

-- 
Regards/Gruss,
    Boris.

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