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

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Date:	Tue, 30 Jun 2015 14:57:36 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Prarit Bhargava <prarit@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
	Len Brown <len.brown@...el.com>,
	Dasaratharaman Chandramouli 
	<dasaratharaman.chandramouli@...el.com>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [PATCH] x86, msr: Allow read access to /dev/cpu/X/msr


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Jun 30, 2015 at 08:20:55AM -0400, Prarit Bhargava wrote:
> > it seems like visiting changes on each of these packages (and the other
> > packages that I'm sure I've missed) will be moderately difficult.
> > 
> > Thoughts?
> 
> Start by changing the ones users want to run most and leave the rest
> requiring root privs until someone has the time to convert them.

Yes, start by just converting the ones used by a single tool, say turbostat, and 
then convert turbostat (while keeping /dev/msr fall-back code as well) just to see 
what the tooling fallout is.

> Its not like we can remove the msr driver any time soon anyway.

Yes.

> So I would suggest starting with the perf MSR driver thingy for all
> those MSRs that count things and see if you can convert say
> turbostat/cpufrequtils/powertop over to that.

One of those tools would be enough, to keep the complexity of the initial 
submission low - and to allow a change of plans if necessary.

> I suspect there's MSR that are useful to expose but are not counting, I'm not 
> sure perf is the right interface for those.
>
> Making an inventory on which MSRs are required by these tools and what kind of 
> data they provide might give a good idea on how to continue.
> 
> If most of these tools only use counting MSRs that can be serviced with the 
> perf-msr driver then that would be great.

Fully agreed!

Thanks,

	Ingo
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