Re: [PATCH] vfio: Remove VFIO_TYPE1_NESTING_IOMMU

From: Alex Williamson <alex.williamson@redhat.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Cornelia Huck <cohuck@redhat.com>,
	iommu@lists.linux-foundation.org, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Robin Murphy <robin.murphy@arm.com>,
	Joerg Roedel <joro@8bytes.org>,
	Nicolin Chen <nicolinc@nvidia.com>, Will Deacon <will@kernel.org>,
	Eric Auger <eric.auger@redhat.com>
Subject: Re: [PATCH] vfio: Remove VFIO_TYPE1_NESTING_IOMMU
Date: Tue, 17 May 2022 14:26:56 -0600	[thread overview]
Message-ID: <20220517142656.140deb10.alex.williamson@redhat.com> (raw)
In-Reply-To: <0-v1-0093c9b0e345+19-vfio_no_nesting_jgg@nvidia.com>

On Tue, 10 May 2022 13:55:24 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> This control causes the ARM SMMU drivers to choose a stage 2
> implementation for the IO pagetable (vs the stage 1 usual default),
> however this choice has no visible impact to the VFIO user. Further qemu
> never implemented this and no other userspace user is known.
> 
> The original description in commit f5c9ecebaf2a ("vfio/iommu_type1: add
> new VFIO_TYPE1_NESTING_IOMMU IOMMU type") suggested this was to "provide
> SMMU translation services to the guest operating system" however the rest
> of the API to set the guest table pointer for the stage 1 was never
> completed, or at least never upstreamed, rendering this part useless dead
> code.
> 
> Since the current patches to enable nested translation, aka userspace page
> tables, rely on iommufd and will not use the enable_nesting()
> iommu_domain_op, remove this infrastructure. However, don't cut too deep
> into the SMMU drivers for now expecting the iommufd work to pick it up -
> we still need to create S2 IO page tables.
> 
> Remove VFIO_TYPE1_NESTING_IOMMU and everything under it including the
> enable_nesting iommu_domain_op.
> 
> Just in-case there is some userspace using this continue to treat
> requesting it as a NOP, but do not advertise support any more.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 16 ----------------
>  drivers/iommu/arm/arm-smmu/arm-smmu.c       | 16 ----------------
>  drivers/iommu/iommu.c                       | 10 ----------
>  drivers/vfio/vfio_iommu_type1.c             | 12 +-----------
>  include/linux/iommu.h                       |  3 ---
>  include/uapi/linux/vfio.h                   |  2 +-
>  6 files changed, 2 insertions(+), 57 deletions(-)
> 
> It would probably make sense for this to go through the VFIO tree with Robin's
> ack for the SMMU changes.

I'd be in favor of applying this, but it seems Robin and Eric are
looking for a stay of execution and I'd also be looking for an ack from
Joerg.  Thanks,

Alex

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