Re: [PATCH v2 1/2] vfio-mdev: Wire in a request handler for mdev parent

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On 12/2/20 3:28 PM, Alex Williamson wrote:
On Fri, 20 Nov 2020 19:07:39 +0100
Eric Farman <farman@xxxxxxxxxxxxx> wrote:

While performing some destructive tests with vfio-ccw, where the
paths to a device are forcible removed and thus the device itself
is unreachable, it is rather easy to end up in an endless loop in
vfio_del_group_dev() due to the lack of a request callback for the
associated device.

In this example, one MDEV (77c) is used by a guest, while another
(77b) is not. The symptom is that the iommu is detached from the
mdev for 77b, but not 77c, until that guest is shutdown:

     [  238.794867] vfio_ccw 0.0.077b: MDEV: Unregistering
     [  238.794996] vfio_mdev 11f2d2bc-4083-431d-a023-eff72715c4f0: Removing from iommu group 2
     [  238.795001] vfio_mdev 11f2d2bc-4083-431d-a023-eff72715c4f0: MDEV: detaching iommu
     [  238.795036] vfio_ccw 0.0.077c: MDEV: Unregistering
     ...silence...

Let's wire in the request call back to the mdev device, so that a
device being physically removed from the host can be (gracefully?)
handled by the parent device at the time the device is removed.

Add a message when registering the device if a driver doesn't
provide this callback, so a clue is given that this same loop
may be encountered in a similar situation.

Signed-off-by: Eric Farman <farman@xxxxxxxxxxxxx>
---
  drivers/vfio/mdev/mdev_core.c |  4 ++++
  drivers/vfio/mdev/vfio_mdev.c | 10 ++++++++++
  include/linux/mdev.h          |  4 ++++
  3 files changed, 18 insertions(+)

diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
index b558d4cfd082..6de97d25a3f8 100644
--- a/drivers/vfio/mdev/mdev_core.c
+++ b/drivers/vfio/mdev/mdev_core.c
@@ -154,6 +154,10 @@ int mdev_register_device(struct device *dev, const struct mdev_parent_ops *ops)
  	if (!dev)
  		return -EINVAL;
+ /* Not mandatory, but its absence could be a problem */
+	if (!ops->request)
+		dev_info(dev, "Driver cannot be asked to release device\n");
+
  	mutex_lock(&parent_list_lock);
/* Check for duplicate */
diff --git a/drivers/vfio/mdev/vfio_mdev.c b/drivers/vfio/mdev/vfio_mdev.c
index 30964a4e0a28..06d8fc4a6d72 100644
--- a/drivers/vfio/mdev/vfio_mdev.c
+++ b/drivers/vfio/mdev/vfio_mdev.c
@@ -98,6 +98,15 @@ static int vfio_mdev_mmap(void *device_data, struct vm_area_struct *vma)
  	return parent->ops->mmap(mdev, vma);
  }
+static void vfio_mdev_request(void *device_data, unsigned int count)
+{
+	struct mdev_device *mdev = device_data;
+	struct mdev_parent *parent = mdev->parent;
+
+	if (parent->ops->request)
+		parent->ops->request(mdev, count);

What do you think about duplicating the count==0 notice in the else
case here?  ie.

	else if (count == 0)
		dev_notice(mdev_dev(mdev), "No mdev vendor driver	request callback support, blocked until released by user\n");


I'm fine with that. If there are no objections, I should be able to spin a v3 with such a change tomorrow.

Thank you!

Eric

This at least puts something in the log a bit closer to the timeframe
of a possible issue versus the registration nag.  vfio-core could do
this too, but vfio-mdev registers a request callback on behalf of all
mdev devices, so vfio-core would no longer have visibility for this
case.

Otherwise this series looks fine to me and I can take it through the
vfio tree.  Thanks,

Alex

+}
+
  static const struct vfio_device_ops vfio_mdev_dev_ops = {
  	.name		= "vfio-mdev",
  	.open		= vfio_mdev_open,
@@ -106,6 +115,7 @@ static const struct vfio_device_ops vfio_mdev_dev_ops = {
  	.read		= vfio_mdev_read,
  	.write		= vfio_mdev_write,
  	.mmap		= vfio_mdev_mmap,
+	.request	= vfio_mdev_request,
  };
static int vfio_mdev_probe(struct device *dev)
diff --git a/include/linux/mdev.h b/include/linux/mdev.h
index 0ce30ca78db0..9004375c462e 100644
--- a/include/linux/mdev.h
+++ b/include/linux/mdev.h
@@ -72,6 +72,9 @@ struct device *mdev_get_iommu_device(struct device *dev);
   * @mmap:		mmap callback
   *			@mdev: mediated device structure
   *			@vma: vma structure
+ * @request:		request callback to release device
+ *			@mdev: mediated device structure
+ *			@count: request sequence number
   * Parent device that support mediated device should be registered with mdev
   * module with mdev_parent_ops structure.
   **/
@@ -92,6 +95,7 @@ struct mdev_parent_ops {
  	long	(*ioctl)(struct mdev_device *mdev, unsigned int cmd,
  			 unsigned long arg);
  	int	(*mmap)(struct mdev_device *mdev, struct vm_area_struct *vma);
+	void	(*request)(struct mdev_device *mdev, unsigned int count);
  };
/* interface for exporting mdev supported type attributes */




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