Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support

[Andy Lutomirski]

On Mon, Oct 8, 2018 at 8:27 AM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
>
> On Sat, Oct 06, 2018 at 03:28:05PM -0700, Andy Lutomirski wrote:
> > On Sat, Oct 6, 2018 at 1:29 PM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Oct 04, 2018 at 03:15:32PM -0700, Andy Lutomirski wrote:
> > > > For better or for worse, I'm trying to understand this code.  So far,
> > > > I've come up with this patch:
> > > >
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso-tglx&id=14fd71e12b1c4492a06f368f75041f263e6862bf
> > > >
> > > > Is it correct, or am I missing some subtlety?
> > >
> > > The master clock, when initialized, has a pair
> > >
> > > masterclockvalues=(TSC value, time-of-day data).
> > >
> > > When updating the guest clock, we only update relative to (TSC value)
> > > that was read on masterclock initialization.
> >
> > I don't see the problem.  The masterclock data is updated here:
> >
> >     host_tsc_clocksource = kvm_get_time_and_clockread(
> >                     &ka->master_kernel_ns,
> >                     &ka->master_cycle_now);
> >
> > kvm_get_time_and_clockread() gets those values from
> > do_monotonic_boot(), which, barring bugs, should cause
> > get_kvmclock_ns() to return exactly the same thing as
> > ktime_get_boot_ns() + ka->kvmclock_offset, albeit in a rather
> > roundabout manner.
> >
> > So what am I missing?  Is there actually something wrong with my patch?
>
> For the bug mentioned in the comment not to happen, you must only read
> TSC and add it as offset to (TSC value, time-of-day data).
>
> Its more than "a roundabout manner".
>
> Read the comment again.
>

I read the comment three more times and even dug through the git
history.  It seems like what you're saying is that, under certain
conditions (which arguably would be bugs in the core Linux timing
code), actually calling ktime_get_boot_ns() could be non-monotonic
with respect to the kvmclock timing.  But get_kvmclock_ns() isn't used
for VM timing as such -- it's used for the IOCTL interfaces for
updating the time offset.  So can you explain how my patch is
incorrect?

--Andy