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