Re: [RFC PATCH 1/2] timekeeper: change interface of clocksource reding functions

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-06-28 13:55+0300, Denis Plotnikov:
> When using timekeepeing API in some cases it is useful to return
> timestamp value used along with the time calculated to use that
> timestamp value for other purpuses (e.g. in KVM master clock timestamp)

Makes sense.  What I don't like about this interface is the TSC centric
approach in a code that isn't even x86 specific -- other architectures
might have a similar counter they'd like to use.

Could we get it a bit more generic?

At least returning the type of the clock and its counter value.
(kvmclock is a bit problematic for the generic solution, because it is
 TSC based, but should pass through the kvmclock value if we were going
 to make it clean ...)

---
Actually, we might be overengineering it.  With the master clock we have
now, I'm thinking that the gtod is not crucial and we could as well be
using something like:

  static bool kvm_get_time_and_clockread(s64 *kernel_ns, u64 *cycle_now)
  {
  	local_irq_save(flags);
  	*cycle_now = rdtsc();
  	*kernel_ns = ktime_get_boot_ns();
  	local_irq_restore(flags);

  	return todo_tsc_hardware_is_good_enough();
  }

because kvmclock is shifted when master clock is recomputed or disabled,
so it should never happen. (And if it happens, then this bit of
imprecision doesn't matter much.)
The host doesn't need to use TSC either, because master clock uses TSC
frequency that differs from boot_ns -- we only need to be sure that the
TSC hardware doesn't change its frequency.

The small delta introduced with this (rdtsc <-> ktime_get_boot_ns) is
the same that the guest would see if we didn't enable master clock.
kvmclock updates are then normally using the same cycle_now/kernel_ns
pair to remain stable.