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

Radim Krcmar
2017-01-13T16:28:09Z

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.

>> > };
>> >
>> > 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.

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