Subject: Re: [PATCH v4 04/25] virtio: defer config changed notifications

From: Rusty Russell <[email-masked]>
Date: Tue, 14 Oct 2014 11:01:12 +1030

"Michael S. Tsirkin" <[email-masked]> writes:
> Defer config changed notifications that arrive during
> probe/scan/freeze/restore.
>
> This will allow drivers to set DRIVER_OK earlier, without worrying about
> racing with config change interrupts.
>
> This change will also benefit old hypervisors (before 2009)
> that send interrupts without checking DRIVER_OK: previously,
> the callback could race with driver-specific initialization.
>
> This will also help simplify drivers.

But AFAICT you never *read* dev->config_changed.

You unconditionally trigger a config_changed event in
virtio_config_enable().  That's a bit weird, but probably OK.

How about the following change (on top of your patch).  I
think the renaming is clearer, and note the added if() test in
virtio_config_enable().

If you approve, I'll fold it in.

Cheers,
Rusty.

diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 2536701b098b..df598dd8c5c8 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -122,7 +122,7 @@ static void __virtio_config_changed(struct virtio_device *dev)
 	struct virtio_driver *drv = drv_to_virtio(dev->dev.driver);
 
 	if (!dev->config_enabled)
-		dev->config_changed = true;
+		dev->config_change_pending = true;
 	else if (drv && drv->config_changed)
 		drv->config_changed(dev);
 }
@@ -148,8 +148,9 @@ static void virtio_config_enable(struct virtio_device *dev)
 {
 	spin_lock_irq(&dev->config_lock);
 	dev->config_enabled = true;
-	__virtio_config_changed(dev);
-	dev->config_changed = false;
+	if (dev->config_change_pending)
+		__virtio_config_changed(dev);
+	dev->config_change_pending = false;
 	spin_unlock_irq(&dev->config_lock);
 }
 
@@ -253,7 +254,7 @@ int register_virtio_device(struct virtio_device *dev)
 
 	spin_lock_init(&dev->config_lock);
 	dev->config_enabled = false;
-	dev->config_changed = false;
+	dev->config_change_pending = false;
 
 	/* We always start by resetting the device, in case a previous
 	 * driver messed it up.  This also tests that code path a little. */
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 5636b119dc25..65261a7244fc 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -80,7 +80,7 @@ bool virtqueue_is_broken(struct virtqueue *vq);
  * @index: unique position on the virtio bus
  * @failed: saved value for CONFIG_S_FAILED bit (for restore)
  * @config_enabled: configuration change reporting enabled
- * @config_changed: configuration change reported while disabled
+ * @config_change_pending: configuration change reported while disabled
  * @config_lock: protects configuration change reporting
  * @dev: underlying device.
  * @id: the device type identification (used to match it with a driver).
@@ -94,7 +94,7 @@ struct virtio_device {
 	int index;
 	bool failed;
 	bool config_enabled;
-	bool config_changed;
+	bool config_change_pending;
 	spinlock_t config_lock;
 	struct device dev;
 	struct virtio_device_id id;