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Date: Fri, 4 Dec 2015 09:28:23 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>, Amy Wiles <amy.l.wiles@intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] x86/rapl: Do not load in a guest
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Fri, Dec 04, 2015 at 08:42:06AM +0100, Ingo Molnar wrote:
> > 
> > * Borislav Petkov <bp@alien8.de> wrote:
> > 
> > > From: Borislav Petkov <bp@suse.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'.

Hm, bad - writing to MSRs like that is generally dangerous.

So we should at least provide a central 'is RAPL available' call instead of 
spreading multiple X86_FEATURE_HYPERVISOR checks.

Thanks,

	Ingo