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

[Jeremy Fitzhardinge <jeremy@goop.org>]

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