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

Lu Baolu
2021-09-29T02:42:22Z

On 9/29/21 10:29 AM, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@linux.intel.com>
>> 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@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.
>>
>
> Not necessarily. NONE is represented by none of dma_user[mode]
> is valid.
>

Fair enough.

Best regards,
baolu