Re: [PATCH v4 02/13] driver core: Set DMA ownership during driver bind/unbind

[Lu Baolu]

Hi Greg,

On 12/22/21 8:47 PM, Greg Kroah-Hartman wrote:
> Which one will actually care about the iommu_device_set_dma_owner()
> call?  All of them?  None of them?  Some of them?
>
> Again, why can't this just happen in the (very few) bus callbacks that
> care about this?  In following patches in this series, you turn off this
> for the pci_dma_configure users, so what is left?  3 odd bus types that
> are not used often.  How well did you test devices of those types with
> this patchset?
>
> It's fine to have "suppress" fields when they are the minority, but here
> it's a_very_  tiny tiny number of actual devices in a system that will
> ever get the chance to have this check happen for them and trigger,
> right?

Thank you for your comments. Current VFIO implementation supports
devices on pci/platform/amba/fls-mc buses for user-space DMA. So only
those buses need to call iommu_device_set/release_dma_owner() in their
dma_configure/cleanup() callbacks.

The "suppress" field is only for a few device drivers (not devices), for
example,

- vfio-pci, a PCI device driver used to bind to a PCI device so that it
  could be assigned for user-space DMA.

Other similar drivers in drivers/vfio are vfio-fsl-mc, vfio-amba and
vfio-platform. These drivers will call
iommu_device_set/release_dma_owner(DMA_OWNER_USER) explicitly when the
device is assigned to user.

The logic is that on the affected buses (pci/platform/amba/fls-mc),

- for non-vfio drivers, bus dma_configure/cleanup() will automatically
  call iommu_device_set_dma_owner(KERNEL) for the device; [This is the
  majority cases.]

- for vfio drivers, the auto-call will be suppressed, and the vfio
  drivers are supposed to call iommu_device_set_dma_owner(USER) before
  device is assigned to the userspace. [This is the rare case.]

The KERNEL and USER conflict will be detected in
iommu_device_set_dma_owner() with a -EBUSY return value. In that case,
the driver binding or device assignment should be aborted.

Best regards,
baolu