Virtualization - Jun 2022 - [PATCH v2 3/5] vfio/iommu_type1: Remove the domain->ops comparison

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.

> From: Robin Murphy <robin.murphy at arm.com>
> Sent: Wednesday, June 22, 2022 3:55 PM
> 
> 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.
> 
This explanation is much clearer! thanks.