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
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