Re: [Xen-devel] [PATCH v2 COLOPre 03/13] libxc/restore: zero ioreq page only one time

[Yang Hongyang <yanghy@xxxxxxxxxxxxxx>]

On 06/09/2015 03:30 PM, Andrew Cooper wrote:
> On 09/06/2015 01:59, Yang Hongyang wrote:
> > On 06/08/2015 06:15 PM, Andrew Cooper wrote:
> > > On 08/06/15 10:58, Yang Hongyang wrote:
> > > > On 06/08/2015 05:46 PM, Andrew Cooper wrote:
> > > > > On 08/06/15 04:43, Yang Hongyang wrote:
> > > > > > ioreq page contains evtchn which will be set when we resume the
> > > > > > secondary vm the first time. The hypervisor will check if the
> > > > > > evtchn is corrupted, so we cannot zero the ioreq page more
> > > > > > than one time.
> > > > > >
> > > > > > The ioreq->state is always STATE_IOREQ_NONE after the vm is
> > > > > > suspended, so it is OK if we only zero it one time.
> > > > > >
> > > > > > Signed-off-by: Yang Hongyang <yanghy@xxxxxxxxxxxxxx>
> > > > > > Signed-off-by: Wen congyang <wency@xxxxxxxxxxxxxx>
> > > > > > CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > > > The issue here is that we are running the restore algorithm over a
> > > > > domain which has already been running in Xen for a while.  This is a
> > > > > brand new usecase, as far as I am aware.
> > > > Exactly.
> > > >
> > > > > Does the qemu process associated with this domain get frozen while the
> > > > > secondary is being reset, or does the process get destroyed and
> > > > > recreated.
> > > > What do you mean by reset? do you mean secondary is suspended at
> > > > checkpoint?
> > > Well - at the point that the buffered records are being processed, we
> > > are in the process of resetting the state of the secondary to match the
> > > primary.
> > Yes, at this point, the qemu process associated with this domain is
> > frozen.
> > the suspend callback will call libxl__qmp_stop(vm_stop() in qemu) to
> > pause
> > qemu. After we processed all records, qemu will be restored with the
> > received
> > state, that's why we add a libxl__qmp_restore(qemu_load_vmstate() in
> > qemu)
> > api to restore qemu with received state. Currently in libxl, qemu only
> > start
> > with the received state, there's no api to load received state while
> > qemu is
> > running for a while.
> Now I consider this more, it is absolutely wrong to not zero the page
> here.  The event channel in the page is not guaranteed to be the same
> between the primary and secondary,
That's why we don't zero it on secondary.

> and we don't want to unexpectedly
> find a pending/in-flight ioreq.
ioreq->state is always STATE_IOREQ_NONE after the vm is suspended, there
should be no pending/in-flight ioreq at checkpoint.

> Either qemu needs to take care of re-initialising the event channels
> back to appropriate values, or Xen should tolerate the channels
> disappearing.
>
> ~Andrew
> .
--
Thanks,
Yang.

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