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RE: [Xen-devel] Suspend/Save a machine under HVM (questions for implementation in Win GPL PV driveres)

  • To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
  • Date: Tue, 26 Feb 2008 23:00:48 +1100
  • Delivery-date: Tue, 26 Feb 2008 04:01:13 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: Ach4apKHkUpEJOpWRqWSpFVSBjrRzAAA8DNIAAACkNA=
  • Thread-topic: [Xen-devel] Suspend/Save a machine under HVM (questions for implementation in Win GPL PV driveres)

> On 26/2/08 11:27, "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
> > *1 what happens if the outstanding requests were:
> > A - Read value from sector 42
> > B - Write value X to sector 42
> > C - Read value from sector 42
> >
> > If all the requests had been completed by the backend but not yet
> > processed on the frontend, then when the operations are
> > operation A is going to return the value of X, instead of whatever
> > previously in sector 42. Maybe this doesn't matter...
> This doesn't happen with Linux I'm pretty sure, as accesses to a
> block are serialised at a higher level.

I think the higher level doesn't have visibility to this...

. Frontend sends A, B & C to Backend
. Backend completes the requests and sends responses to Frontend
. Suspend happens before Frontend notices the responses
. Time passes...
. Resume
. Frontend is torn down on resume
. Frontend reissues uncompleted (as far as it knows) requests A, B & C

Obviously I don't have a very good understanding of the suspend/resume
operation yet, but I can't see anything that waits for responses for
outstanding requests, so I can't see anything that would prevent my
hypothetical situation from occurring. Unless you are referring to
caching when you talk about 'higher level'?

I also can't imagine a situation where it is going to matter though,
except _maybe_ a migrate on a DomU connected to a cluster filesystem...


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