Re: [PATCH 3/6] x86/hyper-v: reenlightenment notifications support

[Vitaly Kuznetsov]

Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:

> On Mon, Dec 11, 2017 at 10:56:33AM +0100, Vitaly Kuznetsov wrote:
>> Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:
>> > On Fri, Dec 08, 2017 at 11:49:57AM +0100, Vitaly Kuznetsov wrote:
>> >> +void register_hv_tsc_update(void (*cb)(void))
>> >> +{
>> >
>> > The function name seems unfortunate.  IMHO such a name suggests
>> > registering a callback on a notifier chain (rather than unconditionally
>> > replacing the old callback), and having no other side effects.
>> 
>> I see, any suggestion? register_hv_reenlightenment_cb? register_hv_tscchange_cb?
>
> IMHO arm_hv_reenlightenment_cb or arm_hv_tscchange_cb would be better,
> but I'm not very good at giving descriptive names.
>

I would probably try to avoid using 'arm' word in x86 code to assist
poor git-greppers :-) And we actually need a pair of functions
(enable/disable). I will probably go with

set_hv_tscchange_cb()
clear_hv_tscchange_cb()

in v2 unless there's a better suggestion.

>> 
>> >
>> >> +	struct hv_reenlightenment_control re_ctrl = {
>> >> +		.vector = HYPERV_REENLIGHTENMENT_VECTOR,
>> >> +		.enabled = 1,
>> >> +		.target_vp = hv_vp_index[smp_processor_id()]
>> >> +	};
>> >> +	struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
>> >> +
>> >> +	if (!(ms_hyperv.features & HV_X64_ACCESS_REENLIGHTENMENT))
>> >> +		return;
>> >
>> > What happens then?  L2 guests keep running with their clocks ticking at
>> > a different speed?
>> >
>> 
>> In reallity this never happens -- in case nested virtualization is
>> supported reenlightenment is also available. In theory, L0 can emulate
>> TSC acceess for forever after migration.
>
> I would think that Hyper-V only started rdtsc emulation if
> TSC_EMULATION_CONTROL was turned on, which wouldn't happen here.
>

Yes, this is the de-facto behavior I observe with WS2016.

> But indeed, normally this shouldn't be a problem.  It may make sense
> just to issue a warning if the feature is unsupported, though.

Will do in v2, thanks.

-- 
  Vitaly