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

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Xen-devel <xen-devel@lists.xen.org>
Subject: Re: [PATCH 06/10] x86/cpuid: Handle leaf 0x6 in guest_cpuid()
Date: Wed, 22 Feb 2017 09:12:56 +0000	[thread overview]
Message-ID: <accd9ac1-3b45-0e13-88a4-93f8526d8f1f@citrix.com> (raw)
In-Reply-To: <3d8e066c-a251-d632-97cb-ad553161eeda@citrix.com>

On 22/02/17 08:23, Andrew Cooper wrote:
> On 22/02/17 07:31, Jan Beulich wrote:
>>>>> On 21.02.17 at 18:40, <andrew.cooper3@citrix.com> wrote:
>>> On 21/02/17 17:25, Jan Beulich wrote:
>>>>>>> On 20.02.17 at 12:00, <andrew.cooper3@citrix.com> 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.)

Linux seems fine without any of this leakage.  I have checked and C/P
state information is still propagated up to Xen.  I will drop the entire
dynamic adjustment.

~Andrew

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