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

[Lu Baolu]

Hi Christoph,

On 2021/11/15 21:27, Christoph Hellwig wrote:
> 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?

"vfio subsystem"

> > 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 .. ?

Yes.

> >   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.

Sure. Thanks!

> > +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.

You are right. This helper is only for vfio which requires IOMMU_API. I
will remove this.

The helpers in patch 1 seem not the same. The driver core (or bus
dma_configure() callback as suggested) will also call them.

> > +	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.

Move mutex_lock/unlock will make the helper implementation easier. :-)
It seems to be common code style in iommu core. For example,
__iommu_attach_group(), __iommu_group_for_each_dev(), etc.

> > +	mutex_lock(&group->mutex);
> > +	owner = group->dma_owner;
> > +	mutex_unlock(&group->mutex);
>
> No need for a lock to read a single scalar.

Adding the lock will make kcasn happy. Jason G also told me that

[citing from his review comment]
"
It is always incorrect to read concurrent data without an annotation
of some kind.

For instance it can cause mis-execution of logic where the compiler is
unaware that a value it loads is allowed to change - ie no 
READ_ONCE/WRITE_ONCE semantic.
"

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

Best regards,
baolu