virtio: reset function

A reset function solves three problems:

1) It allows us to renegotiate features, eg. if we want to upgrade a
   guest driver without rebooting the guest.

2) It gives us a clean way of shutting down virtqueues: after a reset,
   we know that the buffers won't be used by the host, and

3) It helps the guest recover from messed-up drivers.

So we remove the ->shutdown hook, and the only way we now remove
feature bits is via reset.

We leave it to the driver to do the reset before it deletes queues:
the balloon driver, for example, needs to chat to the host in its
remove function.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 13 insertions, 1 deletions

diff --git a/drivers/lguest/lguest_device.c b/drivers/lguest/lguest_device.c
index ced5b44cebce..84f85e23cca7 100644
--- a/drivers/lguest/lguest_device.c
+++ b/drivers/lguest/lguest_device.c
@@ -54,7 +54,7 @@ struct lguest_device {
  *
  * The configuration information for a device consists of one or more
  * virtqueues, a feature bitmaks, and some configuration bytes.  The
- * configuration bytes don't really matter to us: the Launcher set them up, and
+ * configuration bytes don't really matter to us: the Launcher sets them up, and
  * the driver will look at them during setup.
  *
  * A convenient routine to return the device's virtqueue config array:
@@ -139,9 +139,20 @@ static u8 lg_get_status(struct virtio_device *vdev)
 
 static void lg_set_status(struct virtio_device *vdev, u8 status)
 {
+	BUG_ON(!status);
 	to_lgdev(vdev)->desc->status = status;
 }
 
+/* To reset the device, we (ab)use the NOTIFY hypercall, with the descriptor
+ * address of the device.  The Host will zero the status and all the
+ * features. */
+static void lg_reset(struct virtio_device *vdev)
+{
+	unsigned long offset = (void *)to_lgdev(vdev)->desc - lguest_devices;
+
+	hcall(LHCALL_NOTIFY, (max_pfn<<PAGE_SHIFT) + offset, 0, 0);
+}
+
 /*
  * Virtqueues
  *
@@ -279,6 +290,7 @@ static struct virtio_config_ops lguest_config_ops = {
 	.set = lg_set,
 	.get_status = lg_get_status,
 	.set_status = lg_set_status,
+	.reset = lg_reset,
 	.find_vq = lg_find_vq,
 	.del_vq = lg_del_vq,
 };