Re: [Xen-devel] [PATCH] xen: add steal_clock support on x86

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 05/18/2016 11:45 AM, David Vrabel wrote:
> On 18/05/16 16:42, Juergen Gross wrote:
>> On 18/05/16 17:25, Boris Ostrovsky wrote:
>>> On 05/18/2016 10:53 AM, Juergen Gross wrote:
>>>> On 18/05/16 16:46, Boris Ostrovsky wrote:
>>>>> On 05/18/2016 08:15 AM, Juergen Gross wrote:
>>>>>>  }
>>>>>>  
>>>>>> +void __init xen_time_setup_guest(void)
>>>>>> +{
>>>>>> +        pv_time_ops.steal_clock = xen_steal_clock;
>>>>>> +
>>>>>> +        static_key_slow_inc(&paravirt_steal_enabled);
>>>>>> +        /*
>>>>>> +         * We can't set paravirt_steal_rq_enabled as this would require 
>>>>>> the
>>>>>> +         * capability to read another cpu's runstate info.
>>>>>> +         */
>>>>>> +}
>>>>> Won't we be accounting for stolen cycles twice now --- once from
>>>>> steal_account_process_tick()->steal_clock() and second time from
>>>>> do_stolen_accounting()?
>>>> Uuh, yes.
>>>>
>>>> I guess I should rip do_stolen_accounting() out, too? 
>>> I don't think PARAVIRT_TIME_ACCOUNTING is always selected for Xen. If
>> This is easy to accomplish. :-)


I looked at KVM code (PARAVIRT_TIME_ACCOUNTING is not selected there
neither) and in their case that's presumably because stealing accounting
is a CPUID bit, i.e. it might not be supported. In Xen case we always
have this interface.


>>
>>> that's indeed the case then we should ifndef do_stolen_accounting(). Or
>>> maybe check for paravirt_steal_enabled.
>> Is this really a sensible thing to do? There is a mechanism used by KVM
>> to do the stolen accounting. I think we should use it instead of having
>> a second implementation doing the same thing in case the generic one
>> isn't enabled.
> I agree.
>
> Although I don't think selecting PARAVIRT_TIME_ACC' is necessary -- I
> don't think it's essential (or is it?).

Looks like it's useful only if paravirt_steal_rq_enabled, which we don't
support yet.

-boris



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