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

[Marcelo Tosatti]

On Mon, Oct 08, 2018 at 10:38:22AM -0700, Andy Lutomirski wrote:
> 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), 

I don't see that as a bug. Its just a side effect of reading two
different clocks (one is CLOCK_MONOTONIC and the other is TSC),
and using those two clocks to as a "base + offset".

As the comment explains, if you do that, can't guarantee monotonicity.

> 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?

ktime_get_boot_ns() has frequency correction applied, while 
reading masterclock + TSC offset does not.

So the clock reads differ.