RE: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> > Yes, possibly of interest.  But does it work with CONFIG_NO_HZ?
> > (I'm expecting that over time NO_HZ will become widespread
> > for VM OS's, though interested in if you agree.)
> 
> It should work, with CONFIG_NO_HZ, as soon as we come out of 
> a long idle
> (likely due to a timer tick), the timekeeping code will accumulate all
> the skipped ticks.
> 
> If we ever get to non-idle NOHZ, we'll need some extra work here
> (probably lazy accumulation done conditionally in the read path), but
> that's also true for filesystem timestamps. 

OK, sounds good.

> > Also very interested in your thoughts about a variation
> > that returns something similar to a TSC_AUX to notify
> > caller that the underlying reference clock has/may have
> > changed.
> 
> I haven't been following that closely. Personally, experience makes me
> skeptical of workarounds for unsynced TSCs. But I'm sure 
> there's sharper
> folks out there that might make it work. The kernel just requires that
> it *really really* works, and not "mostly" works. :)

This is less a workaround for unsynced TSCs than it
is for VM migration (and maybe also time where a
VM is out-of-context or moved to a different pcpu)
though it could probably
be made to work on unsynced TSC boxes also.
Basically an application needing hi-res profiling
info would do:

nsec1 = clock_gettime2(MONOTONIC,&aux1);
  (time passes)
nsec2 = clock_gettime2(MONOTONIC,&aux2);
if (aux1 != aux2)
   discard_measurement();
else
   use_measurement(nsec2-nsec1);

and system software (hypervisor or kernel or
both) is responsible for ensuring aux value
monotonically increases whenever a different
crystal is used.

Without something like this as a vsyscall,
apps will just use rdtscp (which must be emulated
to work properly across a migration).