Re: [RFC PATCH 17/30] iommufd/device: Add TSM Bind/Unbind for TIO support

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Xu Yilun <yilun.xu@linux.intel.com> writes:

....

> +/**
> + * iommufd_device_tsm_bind - Move a device to TSM Bind state
> + * @idev: device to attach
> + * @vdev_id: Input a IOMMUFD_OBJ_VDEVICE
> + *
> + * This configures for device Confidential Computing(CC), and moves the device
> + * to the TSM Bind state. Once this completes the device is locked down (TDISP
> + * CONFIG_LOCKED or RUN), waiting for guest's attestation.
> + *
> + * This function is undone by calling iommufd_device_tsm_unbind().
> + */
> +int iommufd_device_tsm_bind(struct iommufd_device *idev, u32 vdevice_id)
> +{
> +	struct iommufd_vdevice *vdev;
> +	int rc;
> +
> +	if (!dev_is_pci(idev->dev))
> +		return -ENODEV;
> +
> +	vdev = container_of(iommufd_get_object(idev->ictx, vdevice_id, IOMMUFD_OBJ_VDEVICE),
> +			    struct iommufd_vdevice, obj);
> +	if (IS_ERR(vdev))
> +		return PTR_ERR(vdev);
> +
> +	if (vdev->dev != idev->dev) {
> +		rc = -EINVAL;
> +		goto out_put_vdev;
> +	}
> +
> +	mutex_lock(&idev->igroup->lock);
> +	if (idev->vdev) {
> +		rc = -EEXIST;
> +		goto out_unlock;
> +	}
> +
> +	rc = iommufd_vdevice_tsm_bind(vdev);
> +	if (rc)
> +		goto out_unlock;
> +
> +	idev->vdev = vdev;
> +	refcount_inc(&vdev->obj.users);
> +	mutex_unlock(&idev->igroup->lock);
> +
> +	/*
> +	 * Pairs with iommufd_device_tsm_unbind() - catches caller bugs attempting
> +	 * to destroy a bound device.
> +	 */
> +	refcount_inc(&idev->obj.users);
>

Do we really need this refcount_inc? As I understand it, the objects
aren't being pinned directly. Instead, the reference count seems to be
used more as a way to establish an object hierarchy, ensuring that
objects are freed in the correct order.

In vfio_pci_core_close_device(), you’re decrementing the reference, and
on the iommufd side, we’re covered because the VFIO bind operation takes
a file reference (fget)—so iommufd_fops_release() won’t be called
prematurely.

Wouldn’t it be simpler to skip the reference count increment altogether
and just call tsm_unbind in the virtual device’s destroy callback?
(iommufd_vdevice_destroy())

> +	goto out_put_vdev;
> +
> +out_unlock:
> +	mutex_unlock(&idev->igroup->lock);
> +out_put_vdev:
> +	iommufd_put_object(idev->ictx, &vdev->obj);
> +	return rc;
> +}
> +EXPORT_SYMBOL_NS_GPL(iommufd_device_tsm_bind, "IOMMUFD");

-aneesh