Re: [PATCH v4 03/13] PCI: pci_stub: Suppress kernel DMA ownership auto-claiming

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

On 12/31/21 9:10 AM, Lu Baolu wrote:
> 
> On 12/31/21 8:40 AM, Jason Gunthorpe wrote:
>> On Thu, Dec 30, 2021 at 04:24:14PM -0600, Bjorn Helgaas wrote:
>>
>>> I was speculating that maybe the DMA ownership claiming must be done
>>> *before* the driver's .probe() method?
>>
>> This is correct.
>>
>>> If DMA ownership could be claimed by the .probe() method, we
>>> wouldn't need the new flag in struct device_driver.
>>
>> The other requirement is that every existing driver must claim
>> ownership, so pushing this into the device driver's probe op would
>> require revising almost every driver in Linux...
>>
>> In effect the new flag indicates if the driver will do the DMA
>> ownership claim in it's probe, or should use the default claim the
>> core code does.
>>
>> In almost every case a driver should do a claim. A driver like
>> pci-stub, or a bridge, that doesn't actually operate MMIO on the
>> device would be the exception.
> 
> We still need to call iommu_device_use_dma_api() in bus dma_configure()
> callback. But we can call iommu_device_unuse_dma_api() in the .probe()
> of vfio (and vfio-approved) drivers, so that we don't need the new flag
> anymore.

Oh, wait. I didn't think about the hot-plug case. If we call
iommu_device_use_dma_api() in bus dma_configure() anyway, we can't bind
any (no matter vfio or none-vfio) driver to a device if it's group has
already been assigned to user space. It seems that we can't omit this
flag.

Best regards,
baolu