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

Marcelo Tosatti
2017-01-13T17:51:27Z

On Fri, Jan 13, 2017 at 05:28:09PM +0100, Radim Krcmar wrote:
> 2017-01-13 13:34-0200, Marcelo Tosatti:
> > 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.
>
> Oh, so the "s" means "sub-" for sub-nanosecond precision.

It only counts nanoseconds, how can it be sub nanosecond precise?

> >> > };
> >> >
> >> > 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);
>
> I see, thanks. Makes me wonder why our monotonic time is correct then,
> but that is problably thanks to boot_ns.

The actual starting point of the system_timestamp part of kvmclock
does not matter, all it matters is that it counts in nanoseconds.

> >> 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?
>
> The "s" isn't important, even though I don't think we do anything that
> would justify it, but make use just 8 bytes for both.

Unified.

> Renaming nsec_base is ok, but I'm not sure what tk->tkr_mono.xtime_nsec
> is anymore.

Is the nsec part of tk->xtime_sec. See accumulate_nsecs_to_secs
(which is called from the timer interrupt handler).

/**
* struct timekeeper - Structure holding internal timekeeping values.
* @tkr_mono: The readout base structure for CLOCK_MONOTONIC
* @tkr_raw: The readout base structure for
* CLOCK_MONOTONIC_RAW
* @xtime_sec: Current CLOCK_REALTIME time in seconds
* @ktime_sec: Current CLOCK_MONOTONIC time in seconds
* @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
* @offs_real: Offset clock monotonic -> clock realtime
* @offs_boot: Offset clock monotonic -> clock boottime
* @offs_tai: Offset clock monotonic -> clock tai
* @tai_offset: The current UTC to TAI offset in seconds