[PATCH] xen/xenbus: Add quirk to deal with misconfigured backends.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

A rather annoying and common case is when booting a PVonHVM guest
and exposing the PV KBD and PV VFB - as both of those do not
make any sense. The HVM guest is using the VGA driver and the emulated
keyboard for this. So we provide a very basic quirk framework
(can be expanded in the future) to not wait for 6 minutes for those devices
to initialize - they never wont.

To trigger this, put this in your guest config:

vfb = [ 'vnc=1, vnclisten=0.0.0.0 ,vncunused=1']

instead of this:
vnc=1
vnclisten="0.0.0.0"

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 drivers/xen/xenbus/xenbus_probe_frontend.c |   30 ++++++++++++++++++++++++++++
 1 files changed, 30 insertions(+), 0 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_probe_frontend.c b/drivers/xen/xenbus/xenbus_probe_frontend.c
index f20c5f1..80325cf 100644
--- a/drivers/xen/xenbus/xenbus_probe_frontend.c
+++ b/drivers/xen/xenbus/xenbus_probe_frontend.c
@@ -134,7 +134,31 @@ static int read_backend_details(struct xenbus_device *xendev)
 {
 	return xenbus_read_otherend_details(xendev, "backend-id", "backend");
 }
+static bool skip_if_quirk(struct device *dev, void *data)
+{
+	struct xenbus_device *xendev = to_xenbus_device(dev);
+	struct device_driver *drv = data;
+
+	/* Is this operation limited to a particular driver? */
+	if (drv && (dev->driver != drv))
+		return false;
+
+	if (xen_pv_domain())
+		return false;
+
+	/* With older QEMU, for PVonHVM guests the guest config files could
+	 * contain: vfb = [ 'vnc=1, vnclisten=0.0.0.0 ,vncunused=1']
+	 * which is nonsensical as there is no PV FB (or PV KBD) when
+	 * running as HVM guest. */
 
+	if ((strncmp(xendev->nodename, "device/vkbd", 11) == 0))
+		return true;
+
+	if ((strncmp(xendev->nodename, "device/vfb", 10) == 0))
+		return true;
+
+	return false;
+}
 static int is_device_connecting(struct device *dev, void *data)
 {
 	struct xenbus_device *xendev = to_xenbus_device(dev);
@@ -153,6 +177,12 @@ static int is_device_connecting(struct device *dev, void *data)
 		return 0;
 
 	xendrv = to_xenbus_driver(dev->driver);
+
+	if (skip_if_quirk(dev, data)) {
+		printk(KERN_INFO "XENBUS: (quirk) Skipping : %s\n",
+		       xendev->nodename);
+		return 0;
+	}
 	return (xendev->state < XenbusStateConnected ||
 		(xendev->state == XenbusStateConnected &&
 		 xendrv->is_ready && !xendrv->is_ready(xendev)));
-- 
1.7.7.5