On Thu, Jun 09, 2016 at 12:27:10AM -0300, Marcelo Tosatti wrote:
> On Sun, May 29, 2016 at 08:38:44PM -0300, Marcelo Tosatti wrote:
> > On Thu, May 26, 2016 at 05:49:55PM +0300, Roman Kagan wrote:
> > > The condition to schedule per-VM master clock updates is inversed; as a
> > > result the master clocks are never updated and the kvm_clock drift WRT
> > > host's NTP-disciplined clock (which was the motivation to introduce this
> > > machinery in the first place) still remains.
> > >
> > > Fix it, and reword the comment to make it more apparent what the desired
> > > behavior is.
> > >
> > > Cc: Owen Hofmann <osh@xxxxxxxxxx>
> > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > > Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> > > Signed-off-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
> > > ---
> > > arch/x86/kvm/x86.c | 6 +++---
> > > 1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index c805cf4..d8f591c 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -5810,10 +5810,10 @@ static int pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
> > >
> > > update_pvclock_gtod(tk);
> > >
> > > - /* disable master clock if host does not trust, or does not
> > > - * use, TSC clocksource
> > > + /* only schedule per-VM master clock updates if the host uses TSC and
> > > + * there's at least one VM in need of an update
> > > */
> > > - if (gtod->clock.vclock_mode != VCLOCK_TSC &&
> > > + if (gtod->clock.vclock_mode == VCLOCK_TSC &&
> > > atomic_read(&kvm_guest_has_master_clock) != 0)
> > > queue_work(system_long_wq, &pvclock_gtod_work);
> > >
> > > --
> > > 2.5.5
> >
> > NAK, as stated this leaves clocks out of sync between execution of
> > pvclock_gtod_notify and pvclock_gtod_work execution.
>
> Ok, its not feasible to keep both REF_CLOCK (MSR) and shared memory
> (REF_PAGE) clocks in sync. Even if the frequency correction is the same
> for both, the kernel updates monotonic clock differently than the
> stated frequency that is:
>
> monotonic clock (advertised via vsyscall notifier to use mult/freq pair) != tsc*freq
>
> So the best solution IMO is to:
>
> reads of guest clock = max(shared memory clock, get_kernel_ns() +
> kvmclock_offset)
>
> Where reads of guest clock include: 1) kvm_get_time_and_clockread
> (updates to kvmclock areas), 2) rdmsr(REF_CLOCK).
>
> Unless someone has a better idea, Roman, can you update your patch to
> include such solution? for kvm_get_time_and_clockread, can fast forward
> kvmclock_offset so that
>
> kvmclock_offset + get_kernel_ns() = shared memory clock
Also please update the kvm-unit-test to check both ways (i did here and
what happens is that):
Case1: with frequency correction from vsyscall:
a = read_shared_clock;
b = rdmsr(REF_CLOCK);
fail: a > b
Case2: without frequency correction from vsyscall:
a = rdmsr(REF_CLOCK);
b = read_shared_clock;
fail: b < a
And the frequency correction advertised via syscall is the same as what
get_kernel_ns() is using. Conclusion: update_wall_time() updates to
monotonic clock do not match tsc*freqcorrection.
So there is nothing you can do to keep these clocks in sync
(other then checking what is the largest of them when reading
REF_CLOCK).
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