Re: [PATCH v4 07/13] iommu: Add iommu_at[de]tach_device_shared() for multi-device groups

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

On 12/24/21 10:50 AM, Jason Gunthorpe wrote:
>>> We don't need _USER anymore because iommu_group_set_dma_owner() always
>>> does detatch, and iommu_replace_group_domain() avoids ever reassigning
>>> default_domain. The sepecial USER behavior falls out automatically.
>> This means we will grow more group-centric interfaces. My understanding
>> is the opposite that we should hide the concept of group in IOMMU
>> subsystem, and the device drivers only faces device specific interfaces.
> Ideally group interfaces would be reduced, but in this case VFIO needs
> the group. It has sort of a fundamental problem with its uAPI that
> expects the container is fully setup with a domain at the moment the
> group is attached. So deferring domain setup to when the device is
> available becomes a user visible artifact - and if this is important
> or not is a whole research question that isn't really that important
> for this series.
> 
> We also can't just pull a device out of thin air, a device that hasn't
> been probed() hasn't even had dma_configure called! Let alone the
> lifetime and locking problems with that kind of idea.
> 
> So.. leaving it as a group interface makes the most sense,
> particularly for this series which is really about fixing the sharing
> model in the iommu core and deleting the BUG_ONs.

I feel it makes more sense if we leave the attach_device/group
refactoring patches into a separated series. I will come up with this
new series so that people can review and comment on the real code.

Best regards,
baolu