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

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Message-ID: <20151204101954.GA21177@pd.tnic>
Date:	Fri, 4 Dec 2015 11:19:54 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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>,
	Ingo Molnar <mingo@...hat.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] x86/rapl: Do not load in a guest

+ Paolo.

On Fri, Dec 04, 2015 at 09:28:23AM +0100, Ingo Molnar wrote:
> > > 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.

Well, looks like someone dropped the ball at the CPUID registrar. Other
features have more than one CPUID bit allocated to them, this one
doesn't have a single one.

And since there's no CPUID bit, I don't see any other way to detect the
RAPL presence. Poking at MSRs is a bad idea.

I wonder if we could go and allocate a bit in the kvm-emulated CPUID
leafs which says whether RAPL is supported or not.

Then we can go and check for that leaf on baremetal - if it is not
there, we do the vendor + fms check and if it is there, we know we're in
a guest and whether the guest supports it or not.

Dunno.

On the one hand, it looks like a bit too much to me.

On the other, it could be useful for other future feature checks where
we want baremetal and kvm to be synchronized wrt features and a single
method to be used by the kernel for checking features presence works
both on baremetal and virt.

Just a thought, anyway...

hpa, thoughts?

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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