Re: [PATCH RFC 11/12] iommufd: vfio container FD ioctl compatibility

From: Alex Williamson <alex.williamson@redhat.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Lu Baolu <baolu.lu@linux.intel.com>,
	Chaitanya Kulkarni <chaitanyak@nvidia.com>,
	Cornelia Huck <cohuck@redhat.com>,
	Daniel Jordan <daniel.m.jordan@oracle.com>,
	David Gibson <david@gibson.dropbear.id.au>,
	Eric Auger <eric.auger@redhat.com>,
	iommu@lists.linux-foundation.org,
	Jason Wang <jasowang@redhat.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>,
	Joao Martins <joao.m.martins@oracle.com>,
	Kevin Tian <kevin.tian@intel.com>,
	kvm@vger.kernel.org, Matthew Rosato <mjrosato@linux.ibm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Nicolin Chen <nicolinc@nvidia.com>,
	Niklas Schnelle <schnelle@linux.ibm.com>,
	Shameerali Kolothum Thodi  <shameerali.kolothum.thodi@huawei.com>,
	Yi Liu <yi.l.liu@intel.com>, Keqian Zhu <zhukeqian1@huawei.com>
Subject: Re: [PATCH RFC 11/12] iommufd: vfio container FD ioctl compatibility
Date: Wed, 23 Mar 2022 16:51:25 -0600	[thread overview]
Message-ID: <20220323165125.5efd5976.alex.williamson@redhat.com> (raw)
In-Reply-To: <11-v1-e79cd8d168e8+6-iommufd_jgg@nvidia.com>

On Fri, 18 Mar 2022 14:27:36 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> iommufd can directly implement the /dev/vfio/vfio container IOCTLs by
> mapping them into io_pagetable operations. Doing so allows the use of
> iommufd by symliking /dev/vfio/vfio to /dev/iommufd. Allowing VFIO to
> SET_CONTAINER using a iommufd instead of a container fd is a followup
> series.
> 
> Internally the compatibility API uses a normal IOAS object that, like
> vfio, is automatically allocated when the first device is
> attached.
> 
> Userspace can also query or set this IOAS object directly using the
> IOMMU_VFIO_IOAS ioctl. This allows mixing and matching new iommufd only
> features while still using the VFIO style map/unmap ioctls.
> 
> While this is enough to operate qemu, it is still a bit of a WIP with a
> few gaps to be resolved:
> 
>  - Only the TYPE1v2 mode is supported where unmap cannot punch holes or
>    split areas. The old mode can be implemented with a new operation to
>    split an iopt_area into two without disturbing the iopt_pages or the
>    domains, then unmapping a whole area as normal.
> 
>  - Resource limits rely on memory cgroups to bound what userspace can do
>    instead of the module parameter dma_entry_limit.
> 
>  - VFIO P2P is not implemented. Avoiding the follow_pfn() mis-design will
>    require some additional work to properly expose PFN lifecycle between
>    VFIO and iommfd
> 
>  - Various components of the mdev API are not completed yet
>
>  - Indefinite suspend of SW access (VFIO_DMA_MAP_FLAG_VADDR) is not
>    implemented.
> 
>  - The 'dirty tracking' is not implemented
> 
>  - A full audit for pedantic compatibility details (eg errnos, etc) has
>    not yet been done
> 
>  - powerpc SPAPR is left out, as it is not connected to the iommu_domain
>    framework. My hope is that SPAPR will be moved into the iommu_domain
>    framework as a special HW specific type and would expect power to
>    support the generic interface through a normal iommu_domain.

My overall question here would be whether we can actually achieve a
compatibility interface that has sufficient feature transparency that we
can dump vfio code in favor of this interface, or will there be enough
niche use cases that we need to keep type1 and vfio containers around
through a deprecation process?

The locked memory differences for one seem like something that libvirt
wouldn't want hidden and we have questions regarding support for vaddr
hijacking and different ideas how to implement dirty page tracking, not
to mention the missing features that are currently well used, like p2p
mappings, coherency tracking, mdev, etc.

It seems like quite an endeavor to fill all these gaps, while at the
same time QEMU will be working to move to use iommufd directly in order
to gain all the new features.

Where do we focus attention?  Is symlinking device files our proposal
to userspace and is that something achievable, or do we want to use
this compatibility interface as a means to test the interface and
allow userspace to make use of it for transition, if their use cases
allow it, perhaps eventually performing the symlink after deprecation
and eventual removal of the vfio container and type1 code?  Thanks,

Alex