Messages in this thread | | | Subject | Re: [PATCH] xen: add steal_clock support on x86 | From | Juergen Gross <> | Date | Wed, 18 May 2016 18:00:08 +0200 |
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On 18/05/16 17:53, Boris Ostrovsky wrote: > 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(¶virt_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.
So they added it later and the default is to keep the old behavior.
>>>> 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.
I think the patch is still useful. It is reducing code size and it is removing arch-specific Xen-hack(s). With the patch Xen's solution for arm and x86 is common and the same as for KVM. Adding paravirt_steal_rq_enabled later will be much easier as only one function needs substantial modification.
Juergen
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