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

Lu Baolu
2021-11-16T01:59:42Z

Hi Bjorn,

On 11/16/21 4:38 AM, Bjorn Helgaas wrote:
> On Mon, Nov 15, 2021 at 10:05:42AM +0800, Lu Baolu wrote:
>> From the perspective of who is initiating the device to do DMA, device
>> DMA could be divided into the following types:
>>
>> DMA_OWNER_KERNEL: kernel device driver intiates the DMA
>> DMA_OWNER_USER: userspace device driver intiates the DMA
>
> s/intiates/initiates/ (twice)

Yes.

>
> As your first sentence suggests, the driver doesn't actually
> *initiate* the DMA in either case. One of the drivers programs the
> device, and the *device* initiates the DMA.

You are right. I could rephrase it as:

"From the perspective of who is controlling the device to do DMA ..."

>
>> DMA_OWNER_KERNEL and DMA_OWNER_USER are exclusive for all devices in
>> same iommu group as an iommu group is the smallest granularity of device
>> isolation and protection that the IOMMU subsystem can guarantee.
>
> I think this basically says DMA_OWNER_KERNEL and DMA_OWNER_USER are
> attributes of the iommu_group (not an individual device), and it
> applies to all devices in the iommu_group. Below, you allude to the
> fact that the interfaces are per-device. It's not clear to me why you
> made a per-device interface instead of a per-group interface.

Yes, the attributes are of the iommu_group. We have both per-device and
per-iommu_group interfaces. The former is for device drivers and the
latter is only for vfio who has an iommu_group based iommu abstract.

>> This
>> extends the iommu core to enforce this exclusion when devices are
>> assigned to userspace.
>>
>> Basically two new interfaces are provided:
>>
>> int iommu_device_set_dma_owner(struct device *dev,
>> enum iommu_dma_owner mode, struct file *user_file);
>> void iommu_device_release_dma_owner(struct device *dev,
>> enum iommu_dma_owner mode);
>>
>> Although above interfaces are per-device, DMA owner is tracked per group
>> under the hood. An iommu group cannot have both DMA_OWNER_KERNEL
>> and DMA_OWNER_USER set at the same time. Violation of this assumption
>> fails iommu_device_set_dma_owner().
>>
>> Kernel driver which does DMA have DMA_OWNER_KENREL automatically
>> set/released in the driver binding process (see next patch).
>
> s/DMA_OWNER_KENREL/DMA_OWNER_KERNEL/

Yes. Sorry for the typo.

>
>> Kernel driver which doesn't do DMA should not set the owner type (via a
>> new suppress flag in next patch). Device bound to such driver is considered
>> same as a driver-less device which is compatible to all owner types.
>>
>> Userspace driver framework (e.g. vfio) should set DMA_OWNER_USER for
>> a device before the userspace is allowed to access it, plus a fd pointer to
>> mark the user identity so a single group cannot be operated by multiple
>> users simultaneously. Vice versa, the owner type should be released after
>> the user access permission is withdrawn.

Best regards,
baolu