Re: [PATCH 06/11] iommu: Expose group variants of dma ownership interfaces

[Christoph Hellwig <hch@infradead.org>]

On Mon, Nov 15, 2021 at 10:05:47AM +0800, Lu Baolu wrote:
> The vfio needs to set DMA_OWNER_USER for the entire group when attaching

The vfio subsystem?  driver?

> it to a vfio container. So expose group variants of setting/releasing dma
> ownership for this purpose.
> 
> This also exposes the helper iommu_group_dma_owner_unclaimed() for vfio
> report to userspace if the group is viable to user assignment, for

.. for vfio to report .. ?

>  void iommu_device_release_dma_owner(struct device *dev, enum iommu_dma_owner owner);
> +int iommu_group_set_dma_owner(struct iommu_group *group, enum iommu_dma_owner owner,
> +			      struct file *user_file);
> +void iommu_group_release_dma_owner(struct iommu_group *group, enum iommu_dma_owner owner);

Pleae avoid all these overly long lines.

> +static inline int iommu_group_set_dma_owner(struct iommu_group *group,
> +					    enum iommu_dma_owner owner,
> +					    struct file *user_file)
> +{
> +	return -EINVAL;
> +}
> +
> +static inline void iommu_group_release_dma_owner(struct iommu_group *group,
> +						 enum iommu_dma_owner owner)
> +{
> +}
> +
> +static inline bool iommu_group_dma_owner_unclaimed(struct iommu_group *group)
> +{
> +	return false;
> +}

Why do we need these stubs?  All potential callers should already
require CONFIG_IOMMU_API?  Same for the helpers added in patch 1, btw.

> +	mutex_lock(&group->mutex);
> +	ret = __iommu_group_set_dma_owner(group, owner, user_file);
> +	mutex_unlock(&group->mutex);

> +	mutex_lock(&group->mutex);
> +	__iommu_group_release_dma_owner(group, owner);
> +	mutex_unlock(&group->mutex);

Unless I'm missing something (just skipping over the patches),
the existing callers also take the lock just around these calls,
so we don't really need the __-prefixed lowlevel helpers.

> +	mutex_lock(&group->mutex);
> +	owner = group->dma_owner;
> +	mutex_unlock(&group->mutex);

No need for a lock to read a single scalar.

> +
> +	return owner == DMA_OWNER_NONE;
> +}
> +EXPORT_SYMBOL_GPL(iommu_group_dma_owner_unclaimed);