Date:   Fri, 24 Jun 2022 19:31:47 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Nicolin Chen <nicolinc@...dia.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
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Subject: Re: [PATCH v2 3/5] vfio/iommu_type1: Remove the domain->ops
 comparison

On 2022-06-24 14:16, Jason Gunthorpe wrote:
> On Wed, Jun 22, 2022 at 08:54:45AM +0100, Robin Murphy wrote:
>> 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.
> 
> 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? Everything that's on my radar 
would have drivers all competing for the platform bus - Intel and s390 
are somewhat the odd ones out in that respect, but are also non-issues 
as above. 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.

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.

Thanks,
Robin.

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