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

On 10/10/2009 02:24 AM, Jeremy Fitzhardinge wrote:
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

We only hit this if we have a double migration, otherwise we see p != p0.

Most likely all existing implementations do have a write barrier on the guest entry path, so if we add a read barrier between the two compares, that ensures we're reading from the same cpu again.

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

I think we can update the ABI after verifying all implementations do have a write barrier.

Do not meddle in the internals of kernels, for they are subtle and quick to 

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