Re: [PATCH] x86/kvm: fix condition to update kvm master clocks

[Marcelo Tosatti <mtosatti@redhat.com>]

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@google.com>
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Marcelo Tosatti <mtosatti@redhat.com>
> > Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
> > ---
> >  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