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

[Andy Lutomirski]

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.

Is there a reason to do it this way?