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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
 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


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