Re: [Qemu-devel] [PATCH 1/2] xen-bus: Fix backend state transition on device reset

[Anthony PERARD <anthony.perard@citrix.com>]

On Wed, Aug 21, 2019 at 10:36:40AM +0100, Paul Durrant wrote:
> > diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
> > index e40500242d..982eca4533 100644
> > --- a/hw/xen/xen-bus.c
> > +++ b/hw/xen/xen-bus.c
> > @@ -540,9 +540,11 @@ static void xen_device_backend_changed(void *opaque)
> >      /*
> >       * If the toolstack (or unplug request callback) has set the backend
> >       * state to Closing, but there is no active frontend (i.e. the
> > -     * state is not Connected) then set the backend state to Closed.
> > +     * state is not Connected or Closing) then set the backend state
> > +     * to Closed.
> >       */
> >      if (xendev->backend_state == XenbusStateClosing &&
> > +        xendev->frontend_state != XenbusStateClosing &&
> >          xendev->frontend_state != XenbusStateConnected) {
> >          xen_device_backend_set_state(xendev, XenbusStateClosed);
> 
> Actually, I wonder whether it is better to 'whitelist' here? AFAIK the only valid frontend states whether the backend should set itself 'closed' are 'closed' (i.e. the frontend is finished) and 'initialising' (the frontend was never there).

Let's see, what are the reason backend=Closing?
    - frontend changed to Closing (because it wants to disconnect)
    - toolstack(libxl) or QEMU(unplug request) set the state to Closing,
      but also online to 0.

What should the backend do in both case:
    - frontend alive: backend should wait
        frontend state might be InitWait, Initialised, Connected,
        Closing.
    - frontend not existing or disconnected: backend can skip waiting
      and go to the next step, Closed.
        frontend might be Initialising, Closed.
        But there are also Unknown, Reconfiguring and Reconfigured which
        are probably errors.

So, the whitelist with Closed and Initialising is a good start, but what
about the Unknown state? (QEMU doesn't have backends were the state
Reconfigur* is possible, so they can be mapped to Unknown for now).

Cheers,

-- 
Anthony PERARD