Re: [Xen-devel] [PATCH 06/10] x86/cpuid: Handle leaf 0x6 in guest_cpuid()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/02/17 07:31, Jan Beulich wrote:
>>>> On 21.02.17 at 18:40, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 21/02/17 17:25, Jan Beulich wrote:
>>>>>> On 20.02.17 at 12:00, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> The thermal/performance leaf was previously hidden from HVM guests, but 
>> fully
>>>> visible to PV guests.  Most of the leaf refers to MSR availability, and 
>> there
>>>> is nothing an unprivileged PV guest can do with the information, so hide 
>>>> the
>>>> leaf entirely.
>>>>
>>>> The PV MSR handling logic as minimal support for some thermal/perf 
>>>> operations
>>> ... has ...
>>>
>>>> from the hardware domain, so leak through the implemented subset of 
>>>> features.
>>> Does it make sense to continue to special case PV hwdom here?
>> Being able to play with these MSRs will be actively wrong for HVM
>> context.  It is already fairly wrong for PV context, as nothing prevents
>> you being rescheduled across pcpus while in the middle of a read/write
>> cycle on the MSRs.
> So the MSRs in question are, afaics
> - MSR_IA32_MPERF, MSR_IA32_APERF, MSR_IA32_PERF_CTL (all
>   of which are is_cpufreq_controller() dependent)
> - MSR_IA32_THERM_CONTROL, MSR_IA32_ENERGY_PERF_BIAS
>   (both of which are is_pinned_vcpu() dependent)
> For the latter your argument doesn't apply. For the former, I've
> been wondering for a while whether we shouldn't do away with
> "cpufreq=dom0-kernel".

Hmm.  All good points.  If I can get away without leaking any of this,
that would be ideal.  (Lets see what Linux thinks of such a setup.)

~Andrew

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