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

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Message-ID: <BN9PR11MB52760486306A90A208D7C6768CB59@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Thu, 23 Jun 2022 03:50:22 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Robin Murphy <robin.murphy@....com>,
        Nicolin Chen <nicolinc@...dia.com>
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Subject: RE: [PATCH v2 3/5] vfio/iommu_type1: Remove the domain->ops
 comparison

> From: Robin Murphy <robin.murphy@....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.