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

Lu Baolu
2021-09-29T02:25:57Z

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@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)

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.

>
> Jason
>

Best regards,
baolu