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

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Message-ID: <56616DBF.8040900@redhat.com>
Date:	Fri, 4 Dec 2015 11:41:03 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Borislav Petkov <bp@...en8.de>, 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>
Subject: Re: [PATCH] x86/rapl: Do not load in a guest



On 04/12/2015 11:19, Borislav Petkov wrote:
> + 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.

Yup, this is an issue with RAPL.

> 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.

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.

Paolo

> 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?
> 
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