Re: [patch 1/3] KVM: x86: provide realtime host clock via vsyscall notifiers

Marcelo Tosatti
2017-01-13T16:00:15Z

On Fri, Jan 13, 2017 at 04:18:04PM +0100, Radim Krcmar wrote:
> 2017-01-13 10:01-0200, Marcelo Tosatti:
> > Expose the realtime host clock and save the TSC value
> > used for the clock calculation.
> >
> > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> >
> > ---
> > arch/x86/kvm/x86.c | 38 ++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 38 insertions(+)
> >
> > Index: kvm-ptpdriver/arch/x86/kvm/x86.c
> > ===================================================================
> > --- kvm-ptpdriver.orig/arch/x86/kvm/x86.c 2017-01-13 08:59:03.015895353 -0200
> > +++ kvm-ptpdriver/arch/x86/kvm/x86.c 2017-01-13 09:04:46.581415259 -0200
> > @@ -1139,6 +1139,8 @@
> >
> > u64 boot_ns;
> > u64 nsec_base;
> > + u64 wall_time_sec;
> > + u64 wall_time_snsec;
>
> The leading "s" in "snsec" looks like a copy-paste residue.

Just copying the userspace vsyscall interface.

> > };
> >
> > static struct pvclock_gtod_data pvclock_gtod_data;
> > @@ -1162,6 +1164,9 @@
> > vdata->boot_ns = boot_ns;
> > vdata->nsec_base = tk->tkr_mono.xtime_nsec;
> >
> > + vdata->wall_time_sec = tk->xtime_sec;
> > + vdata->wall_time_snsec = tk->tkr_mono.xtime_nsec;
>
> Using tk->tkr_mono offsets for real time seems wrong -- what happens if
> the real time is half a second shifted from monotonic time?

Both the userspace vsyscall interface and getnstimeofday
use it for realtime clock.

Monotonic clock adds the offset:

vdata->monotonic_time_snsec = tk->tkr_mono.xtime_nsec
+
((u64)tk->wall_to_monotonic.tv_nsec
<< tk->tkr_mono.shift);

> If it's ok, then vdata->nsec_base == vdata->wall_time_snsec, so we don't
> need it.

Just copying the userspace vsyscall interface.

Do you actually want to change the "s" and unify wall_time_snsec with
nsec_base?

> > +
> > write_seqcount_end(&vdata->seq);
> > }
> > #endif
> > @@ -1623,6 +1628,28 @@
> > return mode;
> > }
> >
> > +static int do_realtime(struct timespec *ts, cycle_t *cycle_now)
>
> This is too similar to do_monotonic_boot(), but I don't see a solution
> that is both nice and efficient. :(
>
> (It usually means macros or copying pvclock_gtod_data.)

Yep.