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Date: Fri, 4 Dec 2015 12:53:35 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Amy Wiles <amy.l.wiles@intel.com>,
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        LKML <linux-kernel@vger.kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        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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* Borislav Petkov <bp@alien8.de> wrote:

> On Fri, Dec 04, 2015 at 11:41:03AM +0100, Paolo Bonzini wrote:
> > No, please don't.  Why do you need a wrmsr instead of a rdmsr?  If
> > there's no RAPL domains, the device doesn't load.  On hypervisors,
> > reading random MSRs is generally safe.
> 
> Well, we could not do anything, sure, that's an option too. It would
> only be the annoying error message. Which is
> 
> 	pr_err("no valid rapl domains found in package %d\n", rp->id);
> 
> I guess we can tone that down as apparently it is not an error to
> not have valid rapl domains anymore. Maybe kill it altogether:
> rapl_detect_topology() will propagate the error and the driver won't
> load...

So given than nothing really tells us in a clear way whether RAPL is supported or 
not on that kernel, it might be better to just centralize the 'detect RAPL' 
function, and print "x86/rapl: Feature detected" on bootup. That function can also 
install a synthetic CPUID bit, which all other code could use in a clean fashion.

Since it will be an __init function, there's not much of an overhead argument 
against it.

This way it becomes part of the CPUID infrastructure - and eventually it might 
even grow a real CPUID bit in future CPU models.

and we'll have a lot less RAPL detection muck all around. Win-win.

Thanks,

	Ingo