Re: [PATCH V4 07/10] vfio: Enable cdev noiommu mode under iommufd

[Alex Williamson <alex@shazbot.org>]

On Tue, 14 Apr 2026 14:14:09 -0700
Jacob Pan <jacob.pan@linux.microsoft.com> wrote:

> Now that devices under noiommu mode can bind with IOMMUFD and perform
> IOAS operations, lift restrictions on cdev from VFIO side.
> 
> No IOMMU cdevs are explicitly named with noiommu prefix. e.g.
> 
>     /dev/vfio/
>     |-- 7
>     |-- devices
>     |   `-- noiommu-vfio0
>     `-- vfio

The group interface already does this, so the "7" is not
representative.  In fact, since the no-iommu groups are created as
device are bound, chances are they'd be in sync for a single device, so
this would be 'noiommu-0'.  NB. it's correctly represented in the
subsequent patches.

> 
> Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
> 
> ---
> v4:
>    - Move vfio_device_has_group() related out to 5/10
>    - Keep wait loop in vfio_unregister_group_dev (Jason)
> v3:
>    - Add explict dependency on !GENERIC_ATOMIC64
> v2:
>    - Fix build dependency on IOMMU_SUPPORT
> ---
>  drivers/vfio/Kconfig     |  8 ++++++--
>  drivers/vfio/iommufd.c   |  7 -------
>  drivers/vfio/vfio.h      |  8 +-------
>  drivers/vfio/vfio_main.c | 20 ++++++--------------
>  4 files changed, 13 insertions(+), 30 deletions(-)
> 
> diff --git a/drivers/vfio/Kconfig b/drivers/vfio/Kconfig
> index ceae52fd7586..c013255bf7f1 100644
> --- a/drivers/vfio/Kconfig
> +++ b/drivers/vfio/Kconfig
> @@ -22,8 +22,7 @@ config VFIO_DEVICE_CDEV
>  	  The VFIO device cdev is another way for userspace to get device
>  	  access. Userspace gets device fd by opening device cdev under
>  	  /dev/vfio/devices/vfioX, and then bind the device fd with an iommufd
> -	  to set up secure DMA context for device access.  This interface does
> -	  not support noiommu.
> +	  to set up secure DMA context for device access.
>  
>  	  If you don't know what to do here, say N.
>  
> @@ -63,6 +62,11 @@ endif
>  config VFIO_NOIOMMU
>  	bool "VFIO No-IOMMU support"
>  	depends on VFIO_GROUP
> +	depends on !GENERIC_ATOMIC64 # IOMMU_PT_AMDV1 requires cmpxchg64
> +	select GENERIC_PT
> +	select IOMMU_PT
> +	select IOMMU_PT_AMDV1
> +	depends on IOMMU_SUPPORT

Cosmetic nit, group the depends together.

Noting my previous concern about why we keep group support for
non-container builds, what about making VFIO_GROUP_NOIOMMU and
VFIO_CDEV_NOIOMMU?

Also, vfio no-iommu is traditionally gated on CAP_SYS_RAWIO, but those
tests are all in the vfio group code and not replicated here for cdev,
afaict.  That would relax the usage requirements quite significantly.
Thanks,

Alex

>  	help
>  	  VFIO is built on the ability to isolate devices using the IOMMU.
>  	  Only with an IOMMU can userspace access to DMA capable devices be
> diff --git a/drivers/vfio/iommufd.c b/drivers/vfio/iommufd.c
> index a38d262c6028..26c9c3068c77 100644
> --- a/drivers/vfio/iommufd.c
> +++ b/drivers/vfio/iommufd.c
> @@ -25,10 +25,6 @@ int vfio_df_iommufd_bind(struct vfio_device_file *df)
>  
>  	lockdep_assert_held(&vdev->dev_set->lock);
>  
> -	/* Returns 0 to permit device opening under noiommu mode */
> -	if (vfio_device_is_noiommu(vdev))
> -		return 0;
> -
>  	return vdev->ops->bind_iommufd(vdev, ictx, &df->devid);
>  }
>  
> @@ -58,9 +54,6 @@ void vfio_df_iommufd_unbind(struct vfio_device_file *df)
>  
>  	lockdep_assert_held(&vdev->dev_set->lock);
>  
> -	if (vfio_device_is_noiommu(vdev))
> -		return;
> -
>  	if (vdev->ops->unbind_iommufd)
>  		vdev->ops->unbind_iommufd(vdev);
>  }
> diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> index 9e25605da564..ad9e09f6d095 100644
> --- a/drivers/vfio/vfio.h
> +++ b/drivers/vfio/vfio.h
> @@ -376,19 +376,13 @@ void vfio_init_device_cdev(struct vfio_device *device);
>  
>  static inline int vfio_device_add(struct vfio_device *device)
>  {
> -	/* cdev does not support noiommu device */
> -	if (vfio_device_is_noiommu(device))
> -		return device_add(&device->device);
>  	vfio_init_device_cdev(device);
>  	return cdev_device_add(&device->cdev, &device->device);
>  }
>  
>  static inline void vfio_device_del(struct vfio_device *device)
>  {
> -	if (vfio_device_is_noiommu(device))
> -		device_del(&device->device);
> -	else
> -		cdev_device_del(&device->cdev, &device->device);
> +	cdev_device_del(&device->cdev, &device->device);
>  }
>  
>  int vfio_device_fops_cdev_open(struct inode *inode, struct file *filep);
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 5d7c2d014689..3ae3d34c21cc 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -332,13 +332,15 @@ static int __vfio_register_dev(struct vfio_device *device,
>  	if (!device->dev_set)
>  		vfio_assign_device_set(device, device);
>  
> -	ret = dev_set_name(&device->device, "vfio%d", device->index);
> +	ret = vfio_device_set_group(device, type);
>  	if (ret)
>  		return ret;
>  
> -	ret = vfio_device_set_group(device, type);
> +	/* Just to be safe, expose to user explicitly noiommu cdev node */
> +	ret = dev_set_name(&device->device, "%svfio%d",
> +					   device->noiommu ? "noiommu-" : "", device->index);
>  	if (ret)
> -		return ret;
> +		goto err_out;
>  
>  	/*
>  	 * VFIO always sets IOMMU_CACHE because we offer no way for userspace to
> @@ -359,7 +361,7 @@ static int __vfio_register_dev(struct vfio_device *device,
>  	refcount_set(&device->refcount, 1);
>  
>  	/* noiommu device w/o container may have NULL group */
> -	if (!vfio_device_has_group(device))
> +	if (vfio_device_is_noiommu(device) && !vfio_device_has_group(device))
>  		return 0;
>  
>  	vfio_device_group_register(device);
> @@ -396,16 +398,6 @@ void vfio_unregister_group_dev(struct vfio_device *device)
>  	bool interrupted = false;
>  	long rc;
>  
> -	/*
> -	 * For noiommu devices without a container, thus no dummy group,
> -	 * simply delete and unregister to balance refcount.
> -	 */
> -	if (!vfio_device_has_group(device)) {
> -		vfio_device_del(device);
> -		vfio_device_put_registration(device);
> -		return;
> -	}
> -
>  	/*
>  	 * Prevent new device opened by userspace via the
>  	 * VFIO_GROUP_GET_DEVICE_FD in the group path.