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

Jason Gunthorpe via iommu
2021-09-28T14:07:20Z

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@nvidia.com/
>
> 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)

Jason