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

Konrad Rzeszutek Wilk
2017-01-13T15:41:25Z

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.
>
> > };
> >
> > 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?
>
> If it's ok, then vdata->nsec_base == vdata->wall_time_snsec, so we don't
> need it.
>
> > +
> > 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.)

PV Clock is hypervisor agnostic so both KVM and Xen can use it. Is this clock
interface suppose to follow that?

Thanks.
>
> > +{
> > + struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
> > + unsigned long seq;
> > + int mode;
> > + u64 ns;
> > +
> > + do {
> > + seq = read_seqcount_begin(&gtod->seq);
> > + mode = gtod->clock.vclock_mode;
> > + ts->tv_sec = gtod->wall_time_sec;
> > + ns = gtod->wall_time_snsec;
> > + ns += vgettsc(cycle_now);
> > + ns >>= gtod->clock.shift;
> > + } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
> > +
> > + ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
> > + ts->tv_nsec = ns;
> > +
> > + return mode;
> > +}
> > +
> > /* returns true if host is using tsc clocksource */
> > static bool kvm_get_time_and_clockread(s64 *kernel_ns, cycle_t *cycle_now)
> > {
> > @@ -1632,6 +1659,17 @@
> >
> > return do_monotonic_boot(kernel_ns, cycle_now) == VCLOCK_TSC;
> > }
> > +
> > +/* returns true if host is using tsc clocksource */
> > +static bool kvm_get_walltime_and_clockread(struct timespec *ts,
> > + cycle_t *cycle_now)
> > +{
> > + /* checked again under seqlock below */
> > + if (pvclock_gtod_data.clock.vclock_mode != VCLOCK_TSC)
> > + return false;
> > +
> > + return do_realtime(ts, cycle_now) == VCLOCK_TSC;
> > +}
> > #endif
> >
> > /*
> >
> >
> --
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