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

On Sat, Oct 06, 2018 at 03:28:05PM -0700, Andy Lutomirski wrote:

On Sat, Oct 6, 2018 at 1:29 PM Marcelo Tosatti [off-list ref] wrote:

quoted

On Thu, Oct 04, 2018 at 03:15:32PM -0700, Andy Lutomirski wrote:

quoted

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.

quoted

See the following comment on x86.c:

I read that comment, and it's not obvious to me how it's related.