KVM: x86: introduce get_kvmclock_ns

Introduce a function that reads the exact nanoseconds value that is
provided to the guest in kvmclock.  This crystallizes the notion of
kvmclock as a thin veneer over a stable TSC, that the guest will
(hopefully) convert with NTP.  In other words, kvmclock is *not* a
paravirtualized host-to-guest NTP.

Drop the get_kernel_ns() function, that was used both to get the base
value of the master clock and to get the current value of kvmclock.
The former use is replaced by ktime_get_boot_ns(), the latter is
the purpose of get_kernel_ns().

This also allows KVM to provide a Hyper-V time reference counter that
is synchronized with the time that is computed from the TSC page.

Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kernel/pvclock.c

@@ -80,7 +80,7 @@ cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
 
 	do {
 		version = pvclock_read_begin(src);
-		ret = __pvclock_read_cycles(src);
+		ret = __pvclock_read_cycles(src, rdtsc_ordered());
 		flags = src->flags;
 	} while (pvclock_read_retry(src, version));