Re: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma interfaces

[Jason Gunthorpe]

On Thu, Sep 30, 2021 at 01:09:22PM +1000, David Gibson wrote:

> > The *admin* the one responsible to understand the groups, not the
> > applications. The admin has no idea what a group FD is - they should
> > be looking at the sysfs and seeing the iommu_group directories.
> 
> Not just the admin.  If an app is given two devices in the same group
> to use *both* it must understand that and act accordingly.

Yes, but this is true regardless of what the uAPI is, and for common
app cases where we have a single IO Page table for all devices the app
still doesn't need to care about groups since it can just assign all
devices to the same IO page table and everything works out just fine.

For instance qemu without a vIOMMU does not need to care about
groups. It opens a single iommufd, creates a single IO page table that
maps the guest physical space and assigns every device to that IO page
table. No issue.

Only if qemu is creating a vIOMMU does it need to start to look at the
groups and ensure that the group becomes visible to the guest OS. Here
the group fd doesn't really help anything

Jason