Use of barriers in pvclock ABI

[Jeremy Fitzhardinge]

In Xen, we guarantee that the pv clock record will only ever be updated 
by the current cpu, so there will never be any cross-cpu barrier or 
synchronization issues to consider, nor any chance a vcpu will see its 
own clock record in a partially updated state.

I notice the current kvm implementation is the same.

However, the pvclock_clocksource_read() implementation is 
over-engineered, because it checks for an odd version and uses very 
strong rmb() barriers (which generates either an "lfence" or "lock add 
$0, (%esp)").

If we're happy to guarantee as an ABI issue that the record will never 
be updated cross-cpu, then we can make the barriers simply barrier() and 
just check for (src->version != dst->version).

Is that OK with you, or do you want to leave open the possibility of 
doing cross-cpu time updates?

   J
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