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

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Date:   Wed, 22 Jun 2022 08:54:45 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Nicolin Chen <nicolinc@...dia.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "joro@...tes.org" <joro@...tes.org>,
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Subject: Re: [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.