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

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Message-ID: <YrQRgaHXi1bscYzj@Asurada-Nvidia>
Date:   Thu, 23 Jun 2022 00:47:00 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Robin Murphy <robin.murphy@....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 Thu, Jun 23, 2022 at 03:50:22AM +0000, Tian, Kevin wrote:
> External email: Use caution opening links or attachments
> 
> 
> > 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.

Thanks +1

I've also updated the commit log.