Re: [Xen-devel] [PATCH 1/3] x86/vpmu: Calculate vpmu_enabled() based on vpmu_mode value

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.02.17 at 15:38, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 02/13/2017 07:50 AM, Jan Beulich wrote:
>>>>> On 13.02.17 at 03:29, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> vpmu_enabled() is currently reported based on VPMU_CONTEXT_ALLOCATED
>>> bit of VPMU's flags. This presents a problem on Intel processors where
>>> VPMU context is allocated lazily, during the first access to VPMU MSRs.
>>> With such delayed allocation we, for example, cannot properly report
>>> CPUID's leaf 0xa since it is likely that the leaf is queried by a
>>> guest before the guest attempts ro read or write the MSR.
>>>
>>> Instead of keying vpmu_enabled() off the flags we should compute it
>>> based on vpmu_mode.
>>
>> Doesn't this have its own downsides? What if the mode changes
>> behind the back of a DomU? While certain mode changes are
>> disallowed while there are active users, there's nothing preventing
>> mode or features from changing between a DomU querying CPUID
>> and enabling the vPMU.
> 
> As you said, the mode can't change after a domU VCPU has been 
> initialized, i.e. before the VCPU started running.

I don't understand this last part - how is this related to the vCPU
starting to run? Doesn't at least PV initiate the initialization via
hypercall?

Jan


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