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Jason Gunthorpe says: ==================== This series is the start of adding full DMABUF support to iommufd. Currently it is limited to only work with VFIO's DMABUF exporter. It sits on top of Leon's series to add a DMABUF exporter to VFIO: https://lore.kernel.org/all/20251120-dmabuf-vfio-v9-0-d7f71607f371@nvidia.com/ The existing IOMMU_IOAS_MAP_FILE is enhanced to detect DMABUF fd's, but otherwise works the same as it does today for a memfd. The user can select a slice of the FD to map into the ioas and if the underliyng alignment requirements are met it will be placed in the iommu_domain. Though limited, it is enough to allow a VMM like QEMU to connect MMIO BAR memory from VFIO to an iommu_domain controlled by iommufd. This is used for PCI Peer to Peer support in VMs, and is the last feature that the VFIO type 1 container has that iommufd couldn't do. The VFIO type1 version extracts raw PFNs from VMAs, which has no lifetime control and is a use-after-free security problem. Instead iommufd relies on revokable DMABUFs. Whenever VFIO thinks there should be no access to the MMIO it can shoot down the mapping in iommufd which will unmap it from the iommu_domain. There is no automatic remap, this is a safety protocol so the kernel doesn't get stuck. Userspace is expected to know it is doing something that will revoke the dmabuf and map/unmap it around the activity. Eg when QEMU goes to issue FLR it should do the map/unmap to iommufd. Since DMABUF is missing some key general features for this use case it relies on a "private interconnect" between VFIO and iommufd via the vfio_pci_dma_buf_iommufd_map() call. The call confirms the DMABUF has revoke semantics and delivers a phys_addr for the memory suitable for use with iommu_map(). Medium term there is a desire to expand the supported DMABUFs to include GPU drivers to support DPDK/SPDK type use cases so future series will work to add a general concept of revoke and a general negotiation of interconnect to remove vfio_pci_dma_buf_iommufd_map(). I also plan another series to modify iommufd's vfio_compat to transparently pull a dmabuf out of a VFIO VMA to emulate more of the uAPI of type1. The latest series for interconnect negotation to exchange a phys_addr is: https://lore.kernel.org/r/20251027044712.1676175-1-vivek.kasireddy@intel.com And the discussion for design of revoke is here: https://lore.kernel.org/dri-devel/20250114173103.GE5556@nvidia.com/ ==================== Based on a shared branch with vfio. * iommufd_dmabuf: iommufd/selftest: Add some tests for the dmabuf flow iommufd: Accept a DMABUF through IOMMU_IOAS_MAP_FILE iommufd: Have iopt_map_file_pages convert the fd to a file iommufd: Have pfn_reader process DMABUF iopt_pages iommufd: Allow MMIO pages in a batch iommufd: Allow a DMABUF to be revoked iommufd: Do not map/unmap revoked DMABUFs iommufd: Add DMABUF to iopt_pages vfio/pci: Add vfio_pci_dma_buf_iommufd_map() vfio/nvgrace: Support get_dmabuf_phys vfio/pci: Add dma-buf export support for MMIO regions vfio/pci: Enable peer-to-peer DMA transactions by default vfio/pci: Share the core device pointer while invoking feature functions vfio: Export vfio device get and put registration helpers dma-buf: provide phys_vec to scatter-gather mapping routine PCI/P2PDMA: Document DMABUF model PCI/P2PDMA: Provide an access to pci_p2pdma_map_type() function PCI/P2PDMA: Refactor to separate core P2P functionality from memory allocation PCI/P2PDMA: Simplify bus address mapping API PCI/P2PDMA: Separate the mmap() support from the core logic Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.