hw/xen: do not repeatedly try to create a failing backend device

If xen_backend_device_create() fails to instantiate a device, the XenBus
code will just keep trying over and over again each time the bus is
re-enumerated, as long as the backend appears online and in
XenbusStateInitialising.

The only thing which prevents the XenBus code from recreating duplicates
of devices which already exist, is the fact that xen_device_realize()
sets the backend state to XenbusStateInitWait. If the attempt to create
the device doesn't get *that* far, that's when it will keep getting
retried.

My first thought was to handle errors by setting the backend state to
XenbusStateClosed, but that doesn't work for XenConsole which wants to
*ignore* any device of type != "ioemu" completely.

So, make xen_backend_device_create() *keep* the XenBackendInstance for a
failed device, and provide a new xen_backend_exists() function to allow
xen_bus_type_enumerate() to check whether one already exists before
creating a new one.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/hw/xen/xen-backend.h b/include/hw/xen/xen-backend.h
index aac2fd4..0f01631 100644
--- a/include/hw/xen/xen-backend.h
+++ b/include/hw/xen/xen-backend.h
@@ -33,6 +33,7 @@ XenDevice *xen_backend_get_device(XenBackendInstance *backend);
 void xen_backend_register(const XenBackendInfo *info);
 const char **xen_backend_get_types(unsigned int *nr);
 
+bool xen_backend_exists(const char *type, const char *name);
 void xen_backend_device_create(XenBus *xenbus, const char *type,
                                const char *name, QDict *opts, Error **errp);
 bool xen_backend_try_device_destroy(XenDevice *xendev, Error **errp);