Re: [PATCH V2 05/11] vfio: Allow null group for noiommu without containers

[Mostafa Saleh <smostafa@google.com>]

On Thu, Mar 12, 2026 at 08:56:31AM -0700, Jacob Pan wrote:
> In case of noiommu mode is enabled for VFIO cdev without VFIO container
> nor IOMMUFD provided compatibility container, there is no need to
> create a dummy group. Update the group operations to tolerate null group
> pointer.
> 
> Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
> ---
>  drivers/vfio/group.c | 14 ++++++++++++++
>  drivers/vfio/vfio.h  | 17 +++++++++++++++++
>  2 files changed, 31 insertions(+)
> 
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index 4f15016d2a5f..98f2a4f2ebff 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -381,6 +381,9 @@ int vfio_device_block_group(struct vfio_device *device)
>  	struct vfio_group *group = device->group;
>  	int ret = 0;
>  
> +	if (vfio_null_group_allowed() && !group)
> +		return 0;
> +
>  	mutex_lock(&group->group_lock);
>  	if (group->opened_file) {
>  		ret = -EBUSY;
> @@ -398,6 +401,9 @@ void vfio_device_unblock_group(struct vfio_device *device)
>  {
>  	struct vfio_group *group = device->group;
>  
> +	if (vfio_null_group_allowed() && !group)
> +		return;
> +
>  	mutex_lock(&group->group_lock);
>  	group->cdev_device_open_cnt--;
>  	mutex_unlock(&group->group_lock);
> @@ -589,6 +595,14 @@ static struct vfio_group *vfio_noiommu_group_alloc(struct device *dev,
>  	struct vfio_group *group;
>  	int ret;
>  
> +	/*
> +	 * With noiommu enabled under cdev interface only, there is no need to
> +	 * create a vfio_group if the group based containers are not enabled.
> +	 * The cdev interface is exclusively used for iommufd.
> +	 */
> +	if (vfio_null_group_allowed())
> +		return NULL;
> +
Now vfio_device_set_group() can return NULL when called from
__vfio_register_dev() where the error path calls
vfio_device_remove_group() which I believe would break.

But is that really needed, I feel like this optimization is not worth
the extra effort to add those checks and the possiblity of missing
some. what do you think?

Thanks,
Mostafa

>  	iommu_group = iommu_group_alloc();
>  	if (IS_ERR(iommu_group))
>  		return ERR_CAST(iommu_group);
> diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> index 50128da18bca..838c08077ce2 100644
> --- a/drivers/vfio/vfio.h
> +++ b/drivers/vfio/vfio.h
> @@ -113,6 +113,18 @@ bool vfio_device_has_container(struct vfio_device *device);
>  int __init vfio_group_init(void);
>  void vfio_group_cleanup(void);
>  
> +/*
> + * With noiommu enabled and no containers are supported, allow devices that
> + * don't have a dummy group.
> + */
> +static inline bool vfio_null_group_allowed(void)
> +{
> +	if (vfio_noiommu && (!IS_ENABLED(CONFIG_VFIO_CONTAINER) && !IS_ENABLED(CONFIG_IOMMUFD_VFIO_CONTAINER)))
> +		return true;
> +
> +	return false;
> +}
> +
>  static inline bool vfio_device_is_noiommu(struct vfio_device *vdev)
>  {
>  	return IS_ENABLED(CONFIG_VFIO_NOIOMMU) &&
> @@ -189,6 +201,11 @@ static inline void vfio_group_cleanup(void)
>  {
>  }
>  
> +static inline bool vfio_null_group_allowed(void)
> +{
> +	return false;
> +}
> +
>  static inline bool vfio_device_is_noiommu(struct vfio_device *vdev)
>  {
>  	return false;
> -- 
> 2.34.1
>