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

[Andy Lutomirski]

On Thu, Oct 11, 2018 at 3:28 PM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
>
> On Tue, Oct 09, 2018 at 01:09:42PM -0700, Andy Lutomirski wrote:
> > On Tue, Oct 9, 2018 at 8:28 AM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> > >
> > > 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:
> >
> > > > 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.
> > >
> >
> > Ah, okay, I finally think I see what's going on.  In the kvmclock data
> > exposed to the guest, tsc_shift and tsc_to_system_mul come from
> > tgt_tsc_khz, whereas master_kernel_ns and master_cycle_now come from
> > CLOCK_BOOTTIME.  So the kvmclock and kernel clock drift apart at a
> > rate given by the frequency shift and then suddenly agree again every
> > time the pvclock data is updated.
>
> Yes.
>
> > Is there a reason to do it this way?
>
> Since pvclock updates which update system_timestamp are expensive (must stop all vcpus),
> they should be avoided.
>

Fair enough.

> So only HW TSC counts

makes sense.

>, and used as offset against vcpu's tsc_timestamp.
>

Why don't you just expose CLOCK_MONTONIC_RAW or CLOCK_MONOTONIC_RAW
plus suspend time, though?  Then you would actually be tracking a real
kernel timekeeping mode, and you wouldn't need all this complicated
offsetting work to avoid accidentally going backwards.