Messages in this thread | | | Date | Fri, 4 Dec 2015 11:19:54 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/rapl: Do not load in a guest |
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+ 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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