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

[Robin Murphy]

On 2022-06-16 23:23, Nicolin Chen wrote:
> On Thu, Jun 16, 2022 at 06:40:14AM +0000, Tian, Kevin wrote:
>
> > > The domain->ops validation was added, as a precaution, for mixed-driver
> > > systems. However, at this moment only one iommu driver is possible. So
> > > remove it.
> >
> > It's true on a physical platform. But I'm not sure whether a virtual platform
> > is allowed to include multiple e.g. one virtio-iommu alongside a virtual VT-d
> > or a virtual smmu. It might be clearer to claim that (as Robin pointed out)
> > there is plenty more significant problems than this to solve instead of simply
> > saying that only one iommu driver is possible if we don't have explicit code
> > to reject such configuration. 😊
>
> Will edit this part. Thanks!

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.

Thanks,
Robin.