RE: [Qemu-devel] [PULL 06/25] xen: create xenstore areas for XenDevice-s

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Philippe Mathieu-Daudé <philmd@redhat.com>
> Sent: 01 April 2020 17:14
> To: Anthony PERARD <anthony.perard@citrix.com>; qemu-devel@nongnu.org
> Cc: xen-devel@lists.xenproject.org; Peter Maydell <peter.maydell@linaro.org>; Paul Durrant
> <paul@xen.org>; Markus Armbruster <armbru@redhat.com>
> Subject: Re: [Qemu-devel] [PULL 06/25] xen: create xenstore areas for XenDevice-s
> 
> Hi Anthony, Paul.
> 
> Cc'ing Markus too.
> 
> On 1/14/19 2:51 PM, Anthony PERARD wrote:
> > From: Paul Durrant <paul.durrant@citrix.com>
> >
> > This patch adds a new source module, xen-bus-helper.c, which builds on
> > basic libxenstore primitives to provide functions to create (setting
> > permissions appropriately) and destroy xenstore areas, and functions to
> > 'printf' and 'scanf' nodes therein. The main xen-bus code then uses
> > these primitives [1] to initialize and destroy the frontend and backend
> > areas for a XenDevice during realize and unrealize respectively.
> >
> > The 'xen-block' implementation is extended with a 'get_name' method that
> > returns the VBD number. This number is required to 'name' the xenstore
> > areas.
> >
> > NOTE: An exit handler is also added to make sure the xenstore areas are
> >        cleaned up if QEMU terminates without devices being unrealized.
> >
> > [1] The 'scanf' functions are actually not yet needed, but they will be
> >      needed by code delivered in subsequent patches.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> > Reviewed-by: Anthony Perard <anthony.perard@citrix.com>
> > Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
> > ---
> >   hw/block/xen-block.c            |   9 +
> >   hw/xen/Makefile.objs            |   2 +-
> >   hw/xen/trace-events             |  12 +-
> >   hw/xen/xen-bus-helper.c         | 150 +++++++++++++++
> >   hw/xen/xen-bus.c                | 321 +++++++++++++++++++++++++++++++-
> >   include/hw/xen/xen-bus-helper.h |  39 ++++
> >   include/hw/xen/xen-bus.h        |  12 ++
> >   7 files changed, 540 insertions(+), 5 deletions(-)
> >   create mode 100644 hw/xen/xen-bus-helper.c
> >   create mode 100644 include/hw/xen/xen-bus-helper.h
> >
> [...]
> > +static void xen_device_exit(Notifier *n, void *data)
> > +{
> > +    XenDevice *xendev = container_of(n, XenDevice, exit);
> > +
> > +    xen_device_unrealize(DEVICE(xendev), &error_abort);
> >   }
> >
> >   static void xen_device_realize(DeviceState *dev, Error **errp)
> >   {
> >       XenDevice *xendev = XEN_DEVICE(dev);
> >       XenDeviceClass *xendev_class = XEN_DEVICE_GET_CLASS(xendev);
> > +    XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
> >       const char *type = object_get_typename(OBJECT(xendev));
> >       Error *local_err = NULL;
> >
> > -    trace_xen_device_realize(type);
> > +    if (xendev->frontend_id == DOMID_INVALID) {
> > +        xendev->frontend_id = xen_domid;
> > +    }
> > +
> > +    if (xendev->frontend_id >= DOMID_FIRST_RESERVED) {
> > +        error_setg(errp, "invalid frontend-id");
> > +        goto unrealize;
> > +    }
> > +
> > +    if (!xendev_class->get_name) {
> > +        error_setg(errp, "get_name method not implemented");
> > +        goto unrealize;
> > +    }
> > +
> > +    xendev->name = xendev_class->get_name(xendev, &local_err);
> > +    if (local_err) {
> > +        error_propagate_prepend(errp, local_err,
> > +                                "failed to get device name: ");
> > +        goto unrealize;
> > +    }
> > +
> > +    trace_xen_device_realize(type, xendev->name);
> > +
> > +    xen_device_backend_create(xendev, &local_err);
> > +    if (local_err) {
> > +        error_propagate(errp, local_err);
> > +        goto unrealize;
> > +    }
> > +
> > +    xen_device_frontend_create(xendev, &local_err);
> > +    if (local_err) {
> > +        error_propagate(errp, local_err);
> > +        goto unrealize;
> > +    }
> >
> >       if (xendev_class->realize) {
> >           xendev_class->realize(xendev, &local_err);
> > @@ -72,18 +364,43 @@ static void xen_device_realize(DeviceState *dev, Error **errp)
> >           }
> >       }
> >
> > +    xen_device_backend_printf(xendev, "frontend", "%s",
> > +                              xendev->frontend_path);
> > +    xen_device_backend_printf(xendev, "frontend-id", "%u",
> > +                              xendev->frontend_id);
> > +    xen_device_backend_printf(xendev, "online", "%u", 1);
> > +    xen_device_backend_printf(xendev, "hotplug-status", "connected");
> > +
> > +    xen_device_backend_set_state(xendev, XenbusStateInitWait);
> > +
> > +    xen_device_frontend_printf(xendev, "backend", "%s",
> > +                               xendev->backend_path);
> > +    xen_device_frontend_printf(xendev, "backend-id", "%u",
> > +                               xenbus->backend_id);
> > +
> > +    xen_device_frontend_set_state(xendev, XenbusStateInitialising);
> > +
> > +    xendev->exit.notify = xen_device_exit;
> > +    qemu_add_exit_notifier(&xendev->exit);
> >       return;
> >
> >   unrealize:
> >       xen_device_unrealize(dev, &error_abort);
> 
> It seems if unrealize() fails, the error stored in &local_err is never
> reported. Not sure if this can be improved although.

In this case that's essentially a "don't care". We want to know why the realize failed but if the unrealize fails something is probably pretty seriously screwed (hence the error_abort).

  Paul