RE: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma interfaces

[Tian, Kevin]

> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Wednesday, September 29, 2021 10:22 AM
> 
> On 9/28/21 10:07 PM, Jason Gunthorpe wrote:
> > On Tue, Sep 28, 2021 at 09:35:05PM +0800, Lu Baolu wrote:
> >> Another issue is, when putting a device into user-dma mode, all devices
> >> belonging to the same iommu group shouldn't be bound with a kernel-
> dma
> >> driver. Kevin's prototype checks this by READ_ONCE(dev->driver). This is
> >> not lock safe as discussed below,
> >>
> >> https://lore.kernel.org/linux-
> iommu/20210927130935.GZ964074@xxxxxxxxxx/
> >>
> >> Any guidance on this?
> >
> > Something like this?
> >
> >
> > int iommu_set_device_dma_owner(struct device *dev, enum
> device_dma_owner mode,
> > 			       struct file *user_owner)
> > {
> > 	struct iommu_group *group = group_from_dev(dev);
> >
> > 	spin_lock(&iommu_group->dma_owner_lock);
> > 	switch (mode) {
> > 		case DMA_OWNER_KERNEL:
> > 			if (iommu_group-
> >dma_users[DMA_OWNER_USERSPACE])
> > 				return -EBUSY;
> > 			break;
> > 		case DMA_OWNER_SHARED:
> > 			break;
> > 		case DMA_OWNER_USERSPACE:
> > 			if (iommu_group-
> >dma_users[DMA_OWNER_KERNEL])
> > 				return -EBUSY;
> > 			if (iommu_group->dma_owner_file != user_owner) {
> > 				if (iommu_group-
> >dma_users[DMA_OWNER_USERSPACE])
> > 					return -EPERM;
> > 				get_file(user_owner);
> > 				iommu_group->dma_owner_file =
> user_owner;
> > 			}
> > 			break;
> > 		default:
> > 			spin_unlock(&iommu_group->dma_owner_lock);
> > 			return -EINVAL;
> > 	}
> > 	iommu_group->dma_users[mode]++;
> > 	spin_unlock(&iommu_group->dma_owner_lock);
> > 	return 0;
> > }
> >
> > int iommu_release_device_dma_owner(struct device *dev,
> > 				   enum device_dma_owner mode)
> > {
> > 	struct iommu_group *group = group_from_dev(dev);
> >
> > 	spin_lock(&iommu_group->dma_owner_lock);
> > 	if (WARN_ON(!iommu_group->dma_users[mode]))
> > 		goto err_unlock;
> > 	if (!iommu_group->dma_users[mode]--) {
> > 		if (mode == DMA_OWNER_USERSPACE) {
> > 			fput(iommu_group->dma_owner_file);
> > 			iommu_group->dma_owner_file = NULL;
> > 		}
> > 	}
> > err_unlock:
> > 	spin_unlock(&iommu_group->dma_owner_lock);
> > }
> >
> >
> > Where, the driver core does before probe:
> >
> >     iommu_set_device_dma_owner(dev, DMA_OWNER_KERNEL, NULL)
> >
> > pci_stub/etc does in their probe func:
> >
> >     iommu_set_device_dma_owner(dev, DMA_OWNER_SHARED, NULL)
> >
> > And vfio/iommfd does when a struct vfio_device FD is attached:
> >
> >     iommu_set_device_dma_owner(dev, DMA_OWNER_USERSPACE,
> group_file/iommu_file)
> 
> Really good design. It also helps alleviating some pains elsewhere in
> the iommu core.
> 
> Just a nit comment, we also need DMA_OWNER_NONE which will be set
> when
> the driver core unbinds the driver from the device.
> 

Not necessarily. NONE is represented by none of dma_user[mode]
is valid.