Re: [PATCH v5 01/14] iommu: Add dma ownership management interfaces

[Lu Baolu <baolu.lu@linux.intel.com>]

On 1/6/22 11:46 PM, Jason Gunthorpe wrote:
> On Thu, Jan 06, 2022 at 11:54:06AM +0800, Lu Baolu wrote:
>> On 1/5/22 3:23 AM, Jason Gunthorpe wrote:
>>>>>> The device driver oriented interfaces are,
>>>>>>
>>>>>> 	int iommu_device_use_dma_api(struct device *dev);
>>>>>> 	void iommu_device_unuse_dma_api(struct device *dev);
>>>> Nit, do we care whether it uses the actual DMA API?  Or is it just
>>>> that iommu_device_use_dma_api() tells us the driver may program the
>>>> device to do DMA?
>>> As the main purpose, yes this is all about the DMA API because it
>>> asserts the group domain is the DMA API's domain.
>>>
>>> There is a secondary purpose that has to do with the user/kernel
>>> attack you mentioned above. Maintaining the DMA API domain also
>>> prevents VFIO from allowing userspace to operate any device in the
>>> group which blocks P2P attacks to MMIO of other devices.
>>>
>>> This is why, even if the driver doesn't use DMA, it should still do a
>>> iommu_device_use_dma_api(), except in the special cases where we don't
>>> care about P2P attacks (eg pci-stub, bridges, etc).
>>>
>>
>> By the way, use_dma_api seems hard to read. How about
>>
>> 	iommu_device_use_default_dma()?
> 
> You could just say "use default domain"
> 
> IMHO the way the iommu subsystem has its own wonky language is a
> little troublesome. In the rest of the kernel we call this the DMA
> API, while the iommu subsystem calls the domain that the DMA API uses
> the 'default domain' not the 'DMA API' domain.
> 
> Still, it is probably better to align to the iommu language - just be
> sure to put in the function comment that this API 'allows the driver
> to use the DMA API eg dma_map_sg()'

iommu_device_use_default_domain() reads better. And add some comments
to link "default domain" with "DMA API". Thanks!

> 
> Jason
> 

Best regards,
baolu