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

[Konrad Rzeszutek Wilk]

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 (in Xen 4.2 and lower). We provide a very basic quirk
framework to not wait for 6 minutes for those devices to initialize.

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"

Note: The upstream QEMU can do PV KBD, so we can't just outright ignore
the keyboard driver. Instead we change the timeout for that particular
device to be much less.

[v1: Redo with timeout per Ian and Stefano suggestion]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 drivers/xen/xenbus/xenbus_probe_frontend.c |   39 ++++++++++++++++++++++++++-
 1 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/xenbus/xenbus_probe_frontend.c b/drivers/xen/xenbus/xenbus_probe_frontend.c
index f20c5f1..09da9cf 100644
--- a/drivers/xen/xenbus/xenbus_probe_frontend.c
+++ b/drivers/xen/xenbus/xenbus_probe_frontend.c
@@ -157,7 +157,40 @@ static int is_device_connecting(struct device *dev, void *data)
 		(xendev->state == XenbusStateConnected &&
 		 xendrv->is_ready && !xendrv->is_ready(xendev)));
 }
+static int check_for_quirks(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 - at least with Xen 4.1 and Xen 4.2. */
+
+	if ((strncmp(xendev->nodename, "device/vkbd", 11) == 0))
+		return true;
+
+	if ((strncmp(xendev->nodename, "device/vfb", 10) == 0))
+		return true;
+
+	return false;
+}
+#define QUIRKS_TIMEOUT		30
+#define DEFAULT_TIMEOUT		300
+static unsigned int check_for_timeout_quirks(struct device_driver *drv, unsigned int timeout)
+{
+	if (bus_for_each_dev(&xenbus_frontend.bus, NULL, drv,
+				check_for_quirks))
+		return QUIRKS_TIMEOUT;
+	return timeout;
+}
 static int exists_connecting_device(struct device_driver *drv)
 {
 	return bus_for_each_dev(&xenbus_frontend.bus, NULL, drv,
@@ -211,18 +244,20 @@ static void wait_for_devices(struct xenbus_driver *xendrv)
 	unsigned long start = jiffies;
 	struct device_driver *drv = xendrv ? &xendrv->driver : NULL;
 	unsigned int seconds_waited = 0;
+	unsigned int max_delay = DEFAULT_TIMEOUT;
 
 	if (!ready_to_wait_for_devices || !xen_domain())
 		return;
 
+	max_delay = min(check_for_timeout_quirks(drv, max_delay), max_delay);
 	while (exists_connecting_device(drv)) {
 		if (time_after(jiffies, start + (seconds_waited+5)*HZ)) {
 			if (!seconds_waited)
 				printk(KERN_WARNING "XENBUS: Waiting for "
 				       "devices to initialise: ");
 			seconds_waited += 5;
-			printk("%us...", 300 - seconds_waited);
-			if (seconds_waited == 300)
+			printk("%us...", max_delay - seconds_waited);
+			if (seconds_waited == max_delay)
 				break;
 		}
 
-- 
1.7.7.5

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