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

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 17 Nov 2020 04:21:38 +0100
Eric Farman <farman@linux.ibm.com> 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 hot
> unplug can be (gracefully?) handled by the parent device at the time
> the device is being removed.

I think it makes a lot of sense to give the vendor driver a way to
handle requests.

> 
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> ---
>  drivers/vfio/mdev/vfio_mdev.c | 11 +++++++++++
>  include/linux/mdev.h          |  4 ++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/drivers/vfio/mdev/vfio_mdev.c b/drivers/vfio/mdev/vfio_mdev.c
> index 30964a4e0a28..2dd243f73945 100644
> --- a/drivers/vfio/mdev/vfio_mdev.c
> +++ b/drivers/vfio/mdev/vfio_mdev.c
> @@ -98,6 +98,16 @@ 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 (unlikely(!parent->ops->request))

Hm. Do you think that all drivers should implement a ->request()
callback?

> +		return;
> +	parent->ops->request(mdev, count);
> +}
> +
>  static const struct vfio_device_ops vfio_mdev_dev_ops = {
>  	.name		= "vfio-mdev",
>  	.open		= vfio_mdev_open,