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

Borislav Petkov
2015-12-04T10:20:41Z

+ 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.