Re: [PATCH 1/4] iommu: Add a broken_unmanaged_domain flag in iommu_ops

[Nicolin Chen <nicolinc@nvidia.com>]

Hi Robin.

On Fri, Jan 27, 2023 at 09:58:46PM +0000, Robin Murphy wrote:
> External email: Use caution opening links or attachments
> 
> 
> On 2023-01-27 20:04, Nicolin Chen wrote:
> > Both IOMMU_DOMAIN_UNMANAGED and IOMMU_DOMAIN_DMA require the support
> > of __IOMMU_DOMAIN_PAGING capability, i.e. iommu_map/unmap. However,
> > some older iommu drivers do not fully support that, and these drivers
> > also do not advertise support for dma-iommu.c via IOMMU_DOMAIN_DMA,
> > or use arm_iommu_create_mapping(), so largely their implementations
> > of IOMMU_DOMAIN_UNMANAGED are untested. This means that a user like
> > vfio/iommufd does not likely work with them.
> > 
> > Several of them have obvious problems:
> >    * fsl_pamu_domain.c
> >      Without map/unmap ops in the default_domain_ops, it isn't an
> >      unmanaged domain at all.
> >    * mtk_iommu_v1.c
> >      With a fixed 4M "pagetable", it can only map exactly 4G of
> >      memory, but doesn't set the aperture.
> 
> The aperture is easily fixed (one could argue that what's broken there
> are the ARM DMA ops for assuming every IOMMU has a 32-bit IOVA space and
> not checking).
>
> >    * tegra-gart.c
> >      Its notion of attach/detach and groups has to be a complete lie to
> >      get around all the other API expectations.
> 
> That's true, and the domain is tiny and not isolated from the rest of
> the address space outside the aperture, but the one thing it does do is
> support iommu_map/unmap just fine, which is what this flag is documented
> as saying it doesn't.
> 
> > Some others might work but have never been tested with vfio/iommufd:
> >    * msm_iommu.c
> >    * omap-iommu.c
> >    * tegra-smmu.c
> 
> And yet they all have other in-tree users (GPUs on MSM and Tegra,
> remoteproc on OMAP) that allocate unmanaged domains and use
> iommu_map/unmap just fine, so they're clearly not broken either.
> 
> On the flipside, you're also missing cases like apple-dart, which can
> have broken unmanaged domains by any definition, but only under certain
> conditions (at least it "fails safe" and they will refuse attempts to
> attach anything). I'd also question sprd-iommu, which hardly has a
> generally-useful domain size, and has only just recently gained the
> ability to unmap anything successfully. TBH none of the SoC IOMMUs are
> likely to ever be of interest to VFIO or IOMMUFD, since the only things
> they could assign to userspace are the individual devices - usually
> graphics and media engines - that they're coupled to, whose useful
> functionality tends to depend on clocks, phys, and random other
> low-level stuff that would be somewhere between impractical and
> downright unsafe to attempt to somehow expose as well.

Thanks for all the inputs.

> > Thus, mark all these drivers as having "broken" UNAMANGED domains and
> > add a new device_iommu_unmanaged_supported() API for vfio/iommufd and
> > dma-iommu to refuse to work with these drivers.
> > 
> > Co-developed-by: Jason Gunthorpe <jgg@nvidia.com>
> > Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> > Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> 
> [...]
> 
> > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > index 46e1347bfa22..919a5dbad75b 100644
> > --- a/include/linux/iommu.h
> > +++ b/include/linux/iommu.h
> > @@ -245,6 +245,10 @@ struct iommu_iotlb_gather {
> >    *                    pasid, so that any DMA transactions with this pasid
> >    *                    will be blocked by the hardware.
> >    * @pgsize_bitmap: bitmap of all possible supported page sizes
> > + * @broken_unmanaged_domain: IOMMU_DOMAIN_UNMANAGED is not fully functional; the
> > + *                           driver does not really support iommu_map/unmap, but
> > + *                           uses UNMANAGED domains for the IOMMU API, called by
> > + *                           other SOC drivers.
> 
> "uses UNMANAGED domains for the IOMMU API" is literally the definition
> of unmanaged domains :/
> 
> Some "other SOC drivers" use more of the IOMMU API than VFIO does :/
> 
> Please just add IOMMU_CAP_IOMMUFD to represent whatever the nebulous
> requirements of IOMMUFD actually are (frankly it's no less informative
> than calling domains "broken"), handle that in the drivers you care
> about and have tested, and use device_iommu_capable(). What you're
> describing in this series is a capability, and we have a perfectly good
> API for drivers to express those already. Plus, as demonstrated above, a
> positive capability based on empirical testing will be infinitely more
> robust than a negative one based on guessing.

OK. I can change to IOMMU_CAP_IOMMUFD, and add to the drivers that
are tested. And an IOMMU driver that wants to use IOMMUFD can add
such a CAP later whenever it's ready.

Yet, "IOMMU_CAP_IOMMUFD" would make the VFIO change suspicious, so
perhaps the next version is just one CAP patch + one IOMMUFD patch.
@Jason, any concern?

Thank you
Nicolin

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