linux-kernel - Re: [PATCH] x86/rapl: Do not load in a guest

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Message-ID: <20151204082256.GC17308@twins.programming.kicks-ass.net>
Date:	Fri, 4 Dec 2015 09:22:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Borislav Petkov <bp@...en8.de>, Amy Wiles <amy.l.wiles@...el.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/rapl: Do not load in a guest

On Fri, Dec 04, 2015 at 08:42:06AM +0100, Ingo Molnar wrote:
> 
> * Borislav Petkov <bp@...en8.de> wrote:
> 
> > From: Borislav Petkov <bp@...e.de>
> > 
> > qemu/kvm doesn't support RAPL and RAPL doesn't have a CPUID feature bit
> > so check whether we're in a guest instead.
> 
> So when a hypervisor starts supporting RAPL we'll disable the driver erroneously?
> 
> Isn't there any better method to detect RAPL support?
> 
> So in particular in drivers/powercap/intel_rapl.c there's an enumerated list of 
> CPU models, which is used via a x86_match_cpu() call. That's still not ideal (it 
> does not work on hypervisors for example), but even better would be to detect RAPL 
> support in some other fashion, that does not rely on us statically enumerating CPU 
> models that support it.

RAPL isn't enumerated, the best we could do is attempt to write to one
of the writable MSRs and see if that 'works'.

Also, yuck @ powercap/intel_rapl.c for doing rdmsr_on_cpu() +
wrmsr_on_cpu() all over the place.
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