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

[Paolo Bonzini <pbonzini@redhat.com>]

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

Paolo