Re: [PATCH v5 1/6] timekeeper: introduce extended clocksource reading callback

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, 27 Sep 2017, Paolo Bonzini wrote:
> On 27/09/2017 13:53, Thomas Gleixner wrote:
> >> I think the hook should be specific to x86.  For example it could be an
> >> array of function pointers, indexed by vclock_mode, with the same
> >> semantics as read_with_stamp.
> > I don't think you need that.
> > 
> > The get_time_fn() which is handed in to get_device_system_crossstamp() can
> > convey that information:
> > 
> >                 /*
> >                  * Try to synchronously capture device time and a system
> >                  * counter value calling back into the device driver
> >                  */
> >                 ret = get_time_fn(&xtstamp->device, &system_counterval, ctx);
> >                 if (ret)
> >                         return ret;
> > 
> > So in your case get_time_fn() would be kvmclock or hyperv clock specific
> > and the actual hypercall implementation can return a failure code if the
> > requirements are not met:
> > 
> >    1) host clock source is TSC
> >    2) capturing of host time and TSC is atomic
> 
> So you are suggesting reusing the cross-timestamp hypercall to implement
> nested pvclock.  There are advantages and disadvantages to that.
> 
> With read_with_stamp-like callbacks:
> 
> + running on old KVM or on Hyper-V is supported
> - pvclock_gtod_copy does not go away
> 
> With hypercall-based callbacks on the contrary:
> 
> + KVM can use ktime_get_snapshot for the bare metal case
> - only very new KVM is supported

I don't think that it's an either or decision. 

  get_device_system_crossstamp(get_time_fn, ......)

So you can have specific get_time_fn() implementations for your situation:

   old_kvm_fn()
	retrieve data from pvclock_gtod copy

   new_kvm_fn()
	use hypercall

   hyperv_fn()
	do what must be done

All implementations need a way to tell you:

    1) Host time
    2) Host TSC timestamp which corresponds to #1
    3) Validity

       For old_kvm_fn() pvclock_gtod_data.clock.vclock_mode == VCLOCK_TSC
       For new_kvm_fn() hypercall result
       For hyperv_fn() whatever it takes

Thanks,

	tglx