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/include/asm/pvclock.h

@@ -87,9 +87,10 @@ static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, int shift)
 }
 
 static __always_inline
-cycle_t __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src)
+cycle_t __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
+			      u64 tsc)
 {
-	u64 delta = rdtsc_ordered() - src->tsc_timestamp;
+	u64 delta = tsc - src->tsc_timestamp;
 	cycle_t offset = pvclock_scale_delta(delta, src->tsc_to_system_mul,
 					     src->tsc_shift);
 	return src->system_time + offset;