linux-kernel - Re: [PATCH] x86, msr: Allow read access to /dev/cpu/X/msr

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Date:	Sat, 27 Jun 2015 10:33:55 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Len Brown <len.brown@...el.com>,
	Dasaratharaman Chandramouli 
	<dasaratharaman.chandramouli@...el.com>
Subject: Re: [PATCH] x86, msr: Allow read access to /dev/cpu/X/msr


* Prarit Bhargava <prarit@...hat.com> wrote:

> Customers write system monitoring software for single systems as well as
> clusters.  In load-balancing software it is useful to know how "busy" a
> core is.  Unfortunately the only way to get this data is to run as root,
> or use setcap to allow userspace access for particular programs.  Both of
> these options are clunky at best.
> 
> This patch allows read access to the msr dev files which should be okay.
> No damage can be done by reading the MSR values and it allows non-root
> users to run system monitoring software.
> 
> The turbostat code specifically checks for CAP_SYS_RAWIO, which it
> shouldn't have to and I've removed that code.  Additionally I've modified
> the turbostat man page to remove documentation about configuring
> CAP_SYS_RAW_IO.
> 
> Note: Write access to msr is still restricted with this patch.
> 
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: x86@...nel.org
> Cc: Len Brown <len.brown@...el.com>
> Cc: Prarit Bhargava <prarit@...hat.com>
> Cc: Dasaratharaman Chandramouli <dasaratharaman.chandramouli@...el.com>
> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
> ---
>  arch/x86/kernel/msr.c                 |   11 ++++++++---
>  tools/power/x86/turbostat/turbostat.8 |    8 --------
>  tools/power/x86/turbostat/turbostat.c |   17 -----------------
>  3 files changed, 8 insertions(+), 28 deletions(-)

So what's wrong with exposing them as a simplified PMU driver?

That way we only expose the ones we want to - plus tooling can use all the rich 
perf features that can be used around this. (sampling, counting, call chains, 
etc.)

Thanks,

	Ingo
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