Re: [PATCH 01/11] iommu: Add device dma ownership set/release interfaces

Lu Baolu
2021-11-18T01:17:22Z

On 11/17/21 9:35 PM, Jason Gunthorpe wrote:
> On Wed, Nov 17, 2021 at 01:22:19PM +0800, Lu Baolu wrote:
>> Hi Jason,
>>
>> On 11/16/21 9:46 PM, Jason Gunthorpe wrote:
>>> On Tue, Nov 16, 2021 at 09:57:30AM +0800, Lu Baolu wrote:
>>>> Hi Christoph,
>>>>
>>>> On 11/15/21 9:14 PM, Christoph Hellwig wrote:
>>>>> On Mon, Nov 15, 2021 at 10:05:42AM +0800, Lu Baolu wrote:
>>>>>> +enum iommu_dma_owner {
>>>>>> + DMA_OWNER_NONE,
>>>>>> + DMA_OWNER_KERNEL,
>>>>>> + DMA_OWNER_USER,
>>>>>> +};
>>>>>> +
>>>>>
>>>>>> + enum iommu_dma_owner dma_owner;
>>>>>> + refcount_t owner_cnt;
>>>>>> + struct file *owner_user_file;
>>>>>
>>>>> I'd just overload the ownership into owner_user_file,
>>>>>
>>>>> NULL -> no owner
>>>>> (struct file *)1UL) -> kernel
>>>>> real pointer -> user
>>>>>
>>>>> Which could simplify a lot of the code dealing with the owner.
>>>>>
>>>>
>>>> Yeah! Sounds reasonable. I will make this in the next version.
>>>
>>> It would be good to figure out how to make iommu_attach_device()
>>> enforce no other driver binding as a kernel user without a file *, as
>>> Robin pointed to, before optimizing this.
>>>
>>> This fixes an existing bug where iommu_attach_device() only checks the
>>> group size and is vunerable to a hot plug increasing the group size
>>> after it returns. That check should be replaced by this series's logic
>>> instead.
>>
>> As my my understanding, the essence of this problem is that only the
>> user owner of the iommu_group could attach an UNMANAGED domain to it.
>> If I understand it right, how about introducing a new interface to
>> allocate a user managed domain and storing the user file pointer in it.
>
> For iommu_attach_device() the semantic is simple non-sharing, so there
> is no need for the file * at all, it can just be NULL.

The file * being NULL means the device is only owned by the kernel
driver. Perhaps we can check this pointer in iommu_attach_device() to
avoid using it for user domain attachment.

>
>> Does above help here?
>
> No, iommu_attach_device() is kernel only and should not interact with
> userspace.

The existing iommu_attach_device() allows only for singleton group. As
we have added group ownership attribute, we can enforce this interface
only for kernel domain usage.

>
> I'm also going to see if I can learn what Tegra is doing with
> iommu_attach_group()

Okay! Thank you!

>
> Jason
>

Best regards,
baolu