Re: [PATCH v2 3/5] vfio/iommu_type1: Remove the domain->ops comparison

[Jason Gunthorpe]

On Fri, Jun 24, 2022 at 07:31:47PM +0100, Robin Murphy wrote:

> > > Oh, physical platforms with mixed IOMMUs definitely exist already. The main
> > > point is that while bus_set_iommu still exists, the core code effectively
> > > *does* prevent multiple drivers from registering - even in emulated cases
> > > like the example above, virtio-iommu and VT-d would both try to
> > > bus_set_iommu(&pci_bus_type), and one of them will lose. The aspect which
> > > might warrant clarification is that there's no combination of supported
> > > drivers which claim non-overlapping buses *and* could appear in the same
> > > system - even if you tried to contrive something by emulating, say, VT-d
> > > (PCI) alongside rockchip-iommu (platform), you could still only describe one
> > > or the other due to ACPI vs. Devicetree.
> > 
> > Right, and that is still something we need to protect against with
> > this ops check. VFIO is not checking that the bus's are the same
> > before attempting to re-use a domain.
> > 
> > So it is actually functional and does protect against systems with
> > multiple iommu drivers on different busses.
> 
> But as above, which systems *are* those? 

IDK it seems wrong that the system today will allow different buses to
have different IOMMU drivers and not provide a trivial protection
check.

> FWIW my iommu/bus dev branch has got as far as the final bus ops removal and
> allowing multiple driver registrations, and before it allows that, it does
> now have the common attach check that I sketched out in the previous
> discussion of this.

If you want to put the check in your series that seems fine too, as
long as we get it in the end.

> It's probably also noteworthy that domain->ops is no longer the same
> domain->ops that this code was written to check, and may now be different
> between domains from the same driver.

Yes, the vfio check is not good anymore.

Jason