Re: [RFC 03/20] vfio: Add vfio_[un]register_device()

David Gibson
2021-09-29T03:28:13Z

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On Sun, Sep 19, 2021 at 02:38:31PM +0800, Liu Yi L wrote:
> With /dev/vfio/devices introduced, now a vfio device driver has three
> options to expose its device to userspace:
>
> a) only legacy group interface, for devices which haven't been moved to
> iommufd (e.g. platform devices, sw mdev, etc.);
>
> b) both legacy group interface and new device-centric interface, for
> devices which supports iommufd but also wants to keep backward
> compatibility (e.g. pci devices in this RFC);
>
> c) only new device-centric interface, for new devices which don't carry
> backward compatibility burden (e.g. hw mdev/subdev with pasid);
>
> This patch introduces vfio_[un]register_device() helpers for the device
> drivers to specify the device exposure policy to vfio core. Hence the
> existing vfio_[un]register_group_dev() become the wrapper of the new
> helper functions. The new device-centric interface is described as
> 'nongroup' to differentiate from existing 'group' stuff.
>
> TBD: this patch needs to rebase on top of below series from Christoph in
> next version.
>
> "cleanup vfio iommu_group creation"
>
> Legacy userspace continues to follow the legacy group interface.
>
> Newer userspace can first try the new device-centric interface if the
> device is present under /dev/vfio/devices. Otherwise fall back to the
> group interface.
>
> One open about how to organize the device nodes under /dev/vfio/devices/.
> This RFC adopts a simple policy by keeping a flat layout with mixed devname
> from all kinds of devices. The prerequisite of this model is that devnames
> from different bus types are unique formats:
>
> /dev/vfio/devices/0000:00:14.2 (pci)
> /dev/vfio/devices/PNP0103:00 (platform)
> /dev/vfio/devices/83b8f4f2-509f-382f-3c1e-e6bfe0fa1001 (mdev)

Oof. I really don't think this is a good idea. Ensuring that a
format is "unique" in the sense that it can't collide with any of the
other formats, for *every* value of the parameters on both sides is
actually pretty complicated in general.

I think per-type sub-directories would be helpful here, Jason's
suggestion of just sequential numbers would work as well.

> One alternative option is to arrange device nodes in sub-directories based
> on the device type. But doing so also adds one trouble to userspace. The
> current vfio uAPI is designed to have the user query device type via
> VFIO_DEVICE_GET_INFO after opening the device. With this option the user
> instead needs to figure out the device type before opening the device, to
> identify the sub-directory.

Wouldn't this be up to the operator / configuration, rather than the
actual software though? I would assume that typically the VFIO
program would be pointed at a specific vfio device node file to use,
e.g.
my-vfio-prog -d /dev/vfio/pci/0000:0a:03.1

Or more generally, if you're expecting userspace to know a name in a
uniqu pattern, they can equally well know a "type/name" pair.

> Another tricky thing is that "pdev. vs. mdev"
> and "pci vs. platform vs. ccw,..." are orthogonal categorizations. Need
> more thoughts on whether both or just one category should be used to define
> the sub-directories.
>
> Signed-off-by: Liu Yi L <yi.l.liu@intel.com>
> ---
> drivers/vfio/vfio.c | 137 +++++++++++++++++++++++++++++++++++++++----
> include/linux/vfio.h | 9 +++
> 2 files changed, 134 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index 84436d7abedd..1e87b25962f1 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -51,6 +51,7 @@ static struct vfio {
> struct cdev device_cdev;
> dev_t device_devt;
> struct mutex device_lock;
> + struct list_head device_list;
> struct idr device_idr;
> } vfio;
>
> @@ -757,7 +758,7 @@ void vfio_init_group_dev(struct vfio_device *device, struct device *dev,
> }
> EXPORT_SYMBOL_GPL(vfio_init_group_dev);
>
> -int vfio_register_group_dev(struct vfio_device *device)
> +static int __vfio_register_group_dev(struct vfio_device *device)
> {
> struct vfio_device *existing_device;
> struct iommu_group *iommu_group;
> @@ -794,8 +795,13 @@ int vfio_register_group_dev(struct vfio_device *device)
> /* Our reference on group is moved to the device */
> device->group = group;
>
> - /* Refcounting can't start until the driver calls register */
> - refcount_set(&device->refcount, 1);
> + /*
> + * Refcounting can't start until the driver call register. Don’t
> + * start twice when the device is exposed in both group and nongroup
> + * interfaces.
> + */
> + if (!refcount_read(&device->refcount))

Is there a possible race here with something getting in and
incrementing the refcount between the read and set?

> + refcount_set(&device->refcount, 1);
>
> mutex_lock(&group->device_lock);
> list_add(&device->group_next, &group->device_list);
> @@ -804,7 +810,78 @@ int vfio_register_group_dev(struct vfio_device *device)
>
> return 0;
> }
> -EXPORT_SYMBOL_GPL(vfio_register_group_dev);
> +
> +static int __vfio_register_nongroup_dev(struct vfio_device *device)
> +{
> + struct vfio_device *existing_device;
> + struct device *dev;
> + int ret = 0, minor;
> +
> + mutex_lock(&vfio.device_lock);
> + list_for_each_entry(existing_device, &vfio.device_list, vfio_next) {
> + if (existing_device == device) {
> + ret = -EBUSY;
> + goto out_unlock;

This indicates a bug in the caller, doesn't it? Should it be a BUG or
WARN instead?

> + }
> + }
> +
> + minor = idr_alloc(&vfio.device_idr, device, 0, MINORMASK + 1, GFP_KERNEL);
> + pr_debug("%s - mnior: %d\n", __func__, minor);
> + if (minor < 0) {
> + ret = minor;
> + goto out_unlock;
> + }
> +
> + dev = device_create(vfio.device_class, NULL,
> + MKDEV(MAJOR(vfio.device_devt), minor),
> + device, "%s", dev_name(device->dev));
> + if (IS_ERR(dev)) {
> + idr_remove(&vfio.device_idr, minor);
> + ret = PTR_ERR(dev);
> + goto out_unlock;
> + }
> +
> + /*
> + * Refcounting can't start until the driver call register. Don’t
> + * start twice when the device is exposed in both group and nongroup
> + * interfaces.
> + */
> + if (!refcount_read(&device->refcount))
> + refcount_set(&device->refcount, 1);
> +
> + device->minor = minor;
> + list_add(&device->vfio_next, &vfio.device_list);
> + dev_info(device->dev, "Creates Device interface successfully!\n");
> +out_unlock:
> + mutex_unlock(&vfio.device_lock);
> + return ret;
> +}
> +
> +int vfio_register_device(struct vfio_device *device, u32 flags)
> +{
> + int ret = -EINVAL;
> +
> + device->minor = -1;
> + device->group = NULL;
> + atomic_set(&device->opened, 0);
> +
> + if (flags & ~(VFIO_DEVNODE_GROUP | VFIO_DEVNODE_NONGROUP))
> + return ret;
> +
> + if (flags & VFIO_DEVNODE_GROUP) {
> + ret = __vfio_register_group_dev(device);
> + if (ret)
> + return ret;
> + }
> +
> + if (flags & VFIO_DEVNODE_NONGROUP) {
> + ret = __vfio_register_nongroup_dev(device);
> + if (ret && device->group)
> + vfio_unregister_device(device);
> + }
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(vfio_register_device);
>
> /**
> * Get a reference to the vfio_device for a device. Even if the
> @@ -861,13 +938,14 @@ static struct vfio_device *vfio_device_get_from_name(struct vfio_group *group,
> /*
> * Decrement the device reference count and wait for the device to be
> * removed. Open file descriptors for the device... */
> -void vfio_unregister_group_dev(struct vfio_device *device)
> +void vfio_unregister_device(struct vfio_device *device)
> {
> struct vfio_group *group = device->group;
> struct vfio_unbound_dev *unbound;
> unsigned int i = 0;
> bool interrupted = false;
> long rc;
> + int minor = device->minor;
>
> /*
> * When the device is removed from the group, the group suddenly
> @@ -878,14 +956,20 @@ void vfio_unregister_group_dev(struct vfio_device *device)
> * solve this, we track such devices on the unbound_list to bridge
> * the gap until they're fully unbound.
> */
> - unbound = kzalloc(sizeof(*unbound), GFP_KERNEL);
> - if (unbound) {
> - unbound->dev = device->dev;
> - mutex_lock(&group->unbound_lock);
> - list_add(&unbound->unbound_next, &group->unbound_list);
> - mutex_unlock(&group->unbound_lock);
> + if (group) {
> + /*
> + * If caller hasn't called vfio_register_group_dev(), this
> + * branch is not necessary.
> + */
> + unbound = kzalloc(sizeof(*unbound), GFP_KERNEL);
> + if (unbound) {
> + unbound->dev = device->dev;
> + mutex_lock(&group->unbound_lock);
> + list_add(&unbound->unbound_next, &group->unbound_list);
> + mutex_unlock(&group->unbound_lock);
> + }
> + WARN_ON(!unbound);
> }
> - WARN_ON(!unbound);
>
> vfio_device_put(device);
> rc = try_wait_for_completion(&device->comp);
> @@ -910,6 +994,21 @@ void vfio_unregister_group_dev(struct vfio_device *device)
> }
> }
>
> + /* nongroup interface related cleanup */
> + if (minor >= 0) {
> + mutex_lock(&vfio.device_lock);
> + list_del(&device->vfio_next);
> + device->minor = -1;
> + device_destroy(vfio.device_class,
> + MKDEV(MAJOR(vfio.device_devt), minor));
> + idr_remove(&vfio.device_idr, minor);
> + mutex_unlock(&vfio.device_lock);
> + }
> +
> + /* No need go further if no group. */
> + if (!group)
> + return;
> +
> mutex_lock(&group->device_lock);
> list_del(&device->group_next);
> group->dev_counter--;
> @@ -935,6 +1034,18 @@ void vfio_unregister_group_dev(struct vfio_device *device)
> /* Matches the get in vfio_register_group_dev() */
> vfio_group_put(group);
> }
> +EXPORT_SYMBOL_GPL(vfio_unregister_device);
> +
> +int vfio_register_group_dev(struct vfio_device *device)
> +{
> + return vfio_register_device(device, VFIO_DEVNODE_GROUP);
> +}
> +EXPORT_SYMBOL_GPL(vfio_register_group_dev);
> +
> +void vfio_unregister_group_dev(struct vfio_device *device)
> +{
> + vfio_unregister_device(device);
> +}
> EXPORT_SYMBOL_GPL(vfio_unregister_group_dev);
>
> /**
> @@ -2447,6 +2558,7 @@ static int vfio_init_device_class(void)
>
> mutex_init(&vfio.device_lock);
> idr_init(&vfio.device_idr);
> + INIT_LIST_HEAD(&vfio.device_list);
>
> /* /dev/vfio/devices/$DEVICE */
> vfio.device_class = class_create(THIS_MODULE, "vfio-device");
> @@ -2542,6 +2654,7 @@ static int __init vfio_init(void)
> static void __exit vfio_cleanup(void)
> {
> WARN_ON(!list_empty(&vfio.group_list));
> + WARN_ON(!list_empty(&vfio.device_list));
>
> #ifdef CONFIG_VFIO_NOIOMMU
> vfio_unregister_iommu_driver(&vfio_noiommu_ops);
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index 4a5f3f99eab2..9448b751b663 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -26,6 +26,7 @@ struct vfio_device {
> struct list_head group_next;
> int minor;
> atomic_t opened;
> + struct list_head vfio_next;
> };
>
> /**
> @@ -73,6 +74,14 @@ enum vfio_iommu_notify_type {
> VFIO_IOMMU_CONTAINER_CLOSE = 0,
> };
>
> +/* The device can be opened via VFIO_GROUP_GET_DEVICE_FD */
> +#define VFIO_DEVNODE_GROUP BIT(0)
> +/* The device can be opened via /dev/sys/devices/${DEVICE} */
> +#define VFIO_DEVNODE_NONGROUP BIT(1)
> +
> +extern int vfio_register_device(struct vfio_device *device, u32 flags);
> +extern void vfio_unregister_device(struct vfio_device *device);
> +
> /**
> * struct vfio_iommu_driver_ops - VFIO IOMMU driver callbacks
> */

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson

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