On Fri, Mar 19, 2021 at 04:25:40PM -0600, Alex Williamson wrote:

I've been trying to figure out how, but I think not.  A user can have
multiple devices, each with entirely separate IOMMU contexts.  For each
device, the user can create an mmap of memory to that device and add it
to every other IOMMU context.  That enables peer to peer DMA between
all the devices, across all the IOMMU contexts.  But each individual
device has no direct reference to any IOMMU context other than its own.
A callback on the IOMMU can't reach those other contexts either, there's
no guarantee those other contexts are necessarily managed via the same
vfio IOMMU backend driver.  A notifier is the best I can come up with,
please suggest if you have other ideas.  Thanks,

Indeed, reviewing the set again it seems like we need the notifier
here.