Re: [PATCH v2 01/15] vfio: Add helpers for unifying vfio_device life cycle

[Christoph Hellwig <hch@infradead.org>]

On Wed, Sep 07, 2022 at 12:43:30AM +0000, Tian, Kevin wrote:
> > From: Christoph Hellwig
> > Sent: Tuesday, September 6, 2022 5:42 PM
> > 
> > What is the point?  This adds indirect calls, and actually creates
> > more boilerplate code in the drivers.  i.g. when using this code there
> > is more, and harder to read code.
> 
> The point is to align with struct device life cycle when it's introduced
> to vfio_device. The object is released via put_device() then what would
> be the alternative if the driver doesn't provide a @release callback?
> 
> and with @release then naturally @init is also expected.

No, with a release no @init is expected.  The init method is one
of the major obsfucations here, only topped by the weird
vfio_alloc_device macro.  Yes, that saves about 4 lines of code
in every driver, but places a burden on the struct layout and
very much obsfucated things.  Without vfio_alloc_device and
the init method I think much of this would make a lot more sense.

See the patch below that goes on top of this series to show how
undoing these two would look on mbochs.  It it a slight reduction
lines of code, but more readable and much less churn compared
to the status before this series.

diff --git a/samples/vfio-mdev/mbochs.c b/samples/vfio-mdev/mbochs.c
index df95f25fbc0ede..7f01b335fd4dbd 100644
--- a/samples/vfio-mdev/mbochs.c
+++ b/samples/vfio-mdev/mbochs.c
@@ -505,14 +505,12 @@ static int mbochs_reset(struct mdev_state *mdev_state)
 	return 0;
 }
 
-static int mbochs_init_dev(struct vfio_device *vdev)
+static int mbochs_probe(struct mdev_device *mdev)
 {
-	struct mdev_state *mdev_state =
-		container_of(vdev, struct mdev_state, vdev);
-	struct mdev_device *mdev = to_mdev_device(vdev->dev);
+	int avail_mbytes = atomic_read(&mbochs_avail_mbytes);
 	const struct mbochs_type *type =
 		&mbochs_types[mdev_get_type_group_id(mdev)];
-	int avail_mbytes = atomic_read(&mbochs_avail_mbytes);
+	struct mdev_state *mdev_state;
 	int ret = -ENOMEM;
 
 	do {
@@ -521,10 +519,14 @@ static int mbochs_init_dev(struct vfio_device *vdev)
 	} while (!atomic_try_cmpxchg(&mbochs_avail_mbytes, &avail_mbytes,
 				     avail_mbytes - type->mbytes));
 
-	mdev_state->vconfig = kzalloc(MBOCHS_CONFIG_SPACE_SIZE, GFP_KERNEL);
-	if (!mdev_state->vconfig)
+	mdev_state = kzalloc(sizeof(struct mdev_state), GFP_KERNEL);
+	if (mdev_state == NULL)
 		goto err_avail;
 
+	mdev_state->vconfig = kzalloc(MBOCHS_CONFIG_SPACE_SIZE, GFP_KERNEL);
+	if (mdev_state->vconfig == NULL)
+		goto err_state;
+
 	mdev_state->memsize = type->mbytes * 1024 * 1024;
 	mdev_state->pagecount = mdev_state->memsize >> PAGE_SHIFT;
 	mdev_state->pages = kcalloc(mdev_state->pagecount,
@@ -546,38 +548,33 @@ static int mbochs_init_dev(struct vfio_device *vdev)
 	mbochs_create_config_space(mdev_state);
 	mbochs_reset(mdev_state);
 
+	ret = vfio_init_device(&mdev_state->vdev, &mdev->dev, &mbochs_dev_ops);
+	if (ret)
+		goto err_mem;
+
+	ret = vfio_register_emulated_iommu_dev(&mdev_state->vdev);
+	if (ret) {
+		vfio_put_device(&mdev_state->vdev);
+		return ret;
+	}
+
 	dev_info(vdev->dev, "%s: %s, %d MB, %ld pages\n", __func__,
 		 type->name, type->mbytes, mdev_state->pagecount);
+
+	dev_set_drvdata(&mdev->dev, mdev_state);
 	return 0;
 
+err_mem:
+	kfree(mdev_state->pages);
 err_vconfig:
 	kfree(mdev_state->vconfig);
+err_state:
+	kfree(mdev_state);
 err_avail:
 	atomic_add(type->mbytes, &mbochs_avail_mbytes);
 	return ret;
 }
 
-static int mbochs_probe(struct mdev_device *mdev)
-{
-	struct mdev_state *mdev_state;
-	int ret = -ENOMEM;
-
-	mdev_state = vfio_alloc_device(mdev_state, vdev, &mdev->dev,
-				       &mbochs_dev_ops);
-	if (IS_ERR(mdev_state))
-		return PTR_ERR(mdev_state);
-
-	ret = vfio_register_emulated_iommu_dev(&mdev_state->vdev);
-	if (ret)
-		goto err_put_vdev;
-	dev_set_drvdata(&mdev->dev, mdev_state);
-	return 0;
-
-err_put_vdev:
-	vfio_put_device(&mdev_state->vdev);
-	return ret;
-}
-
 static void mbochs_release_dev(struct vfio_device *vdev)
 {
 	struct mdev_state *mdev_state =
@@ -585,7 +582,7 @@ static void mbochs_release_dev(struct vfio_device *vdev)
 
 	kfree(mdev_state->pages);
 	kfree(mdev_state->vconfig);
-	vfio_free_device(vdev);
+	kfree(vdev);
 	atomic_add(mdev_state->type->mbytes, &mbochs_avail_mbytes);
 }
 
@@ -1414,7 +1411,6 @@ static struct attribute_group *mdev_type_groups[] = {
 
 static const struct vfio_device_ops mbochs_dev_ops = {
 	.close_device = mbochs_close_device,
-	.init = mbochs_init_dev,
 	.release = mbochs_release_dev,
 	.read = mbochs_read,
 	.write = mbochs_write,