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

Jeremy Fitzhardinge
2009-10-10T00:51:55Z

On 10/07/09 03:25, Avi Kivity wrote:
> def try_pvclock_vtime():
> tsc, p0 = rdtscp()
> v0 = pvclock[p0].version
> tsc, p = rdtscp()
> t = pvclock_time(pvclock[p], tsc)
> if p != p0 or pvclock[p].version != v0:
> raise Exception("Processor or timebased change under our feet")
> return t

This doesn't quite work.

If we end up migrating some time after the first rdtscp, then the
accesses to pvclock[] will be cross-cpu. Since we don't made any strong
SMP memory ordering guarantees on updating the structure, the snapshot
isn't guaranteed to be consistent even if we re-check the version at the
end.

So to use rdtscp we need to either redefine the update of
pvclock_vcpu_time_info to be SMP-safe, or keep the additional migration
check.

J