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

Rusty Russell
2014-10-14T00:31:12Z

"Michael S. Tsirkin" <mst@redhat.com> 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;