Provide a new two step DMA mapping API

Currently the only efficient way to map a complex memory description through the DMA API is by using the scatterlist APIs. The SG APIs are unique in that they efficiently combine the two fundamental operations of sizing and allocating a large IOVA window from the IOMMU and processing all the per-address swiotlb/flushing/p2p/map details.

This uniqueness has been a long standing pain point as the scatterlist API is mandatory, but expensive to use. It prevents any kind of optimization or feature improvement (such as avoiding struct page for P2P) due to the impossibility of improving the scatterlist.

Several approaches have been explored to expand the DMA API with additional scatterlist-like structures (BIO[1], rlist[2]), instead split up the DMA API to allow callers to bring their own data structure.

The API is split up into parts:
- Allocate IOVA space:
To do any pre-allocation required. This is done based on the caller supplying some details about how much IOMMU address space it would need in worst case.
- Map and unmap relevant structures to pre-allocated IOVA space:
Perform the actual mapping into the pre-allocated IOVA. This is very similar to dma_map_page().

In this and the next series [1], examples of three different users are converted to the new API to show the benefits and its versatility. Each user has a unique flow:
1. RDMA ODP is an example of "SVA mirroring" using HMM that needs to dynamically map/unmap large numbers of single pages. This becomes significantly faster in the IOMMU case as the map/unmap is now just a page table walk, the IOVA allocation is pre-computed once. Significant amounts of memory are saved as there is no longer a need to store the dma_addr_t of each page.
2. VFIO PCI live migration code is building a very large "page list" for the device. Instead of allocating a scatter list entry per allocated page it can just allocate an array of 'struct page *', saving a large amount of memory.
3. NVMe PCI demonstrates how a BIO can be converted to a HW scatter list without having to allocate then populate an intermediate SG table.

To make the use of the new API easier, HMM and block subsystems are extended to hide the optimization details from the caller. Among these optimizations:
* Memory reduction as in most real use cases there is no need to store mapped DMA addresses and unmap them.
* Reducing the function call overhead by removing the need to call function pointers and use direct calls instead.

This step is first along a path to provide alternatives to scatterlist and solve some of the abuses and design mistakes, for instance in DMABUF's P2P support.

Thanks

[1] https://lore.kernel.org/all/[email protected]

Christoph Hellwig (6):
PCI/P2PDMA: refactor the p2pdma mapping helpers
dma-mapping: move the PCI P2PDMA mapping helpers to pci-p2pdma.h
iommu: generalize the batched sync after map interface iommu/dma: Factor out a iommu_dma_map_swiotlb helper
dma-mapping: add a dma_need_unmap helper
docs: core-api: document the IOVA-based API

Leon Romanovsky (12):
dma-mapping: Add check if IOVA can be used
dma: Provide an interface to allow allocate IOVA
dma-mapping: Implement link/unlink ranges API
mm/hmm: let users to tag specific PFN with DMA mapped bit mm/hmm: provide generic DMA managing logic
RDMA/umem: Store ODP access mask information in PFN RDMA/core: Convert UMEM ODP DMA mapping to caching IOVA and page
linkage
RDMA/umem: Separate implicit ODP initialization from explicit ODP vfio/mlx5: Explicitly use number of pages instead of allocated length vfio/mlx5: Rewrite create mkey flow to allow better code reuse vfio/mlx5: Explicitly store page list
vfio/mlx5: Convert vfio to use DMA link API

 Documentation/core-api/dma-api.rst   |  70 +++++
 drivers/infiniband/core/umem_odp.c   | 250 +++++----------
 drivers/infiniband/hw/mlx5/mlx5_ib.h |  12 +-
 drivers/infiniband/hw/mlx5/odp.c     |  65 ++--
 drivers/infiniband/hw/mlx5/umr.c     |  12 +-
 drivers/iommu/dma-iommu.c            | 455 +++++++++++++++++++++++----
 drivers/iommu/iommu.c                |  65 ++--
 drivers/pci/p2pdma.c                 |  38 +--
 drivers/vfio/pci/mlx5/cmd.c          | 312 +++++++++---------
 drivers/vfio/pci/mlx5/cmd.h          |  24 +-
 drivers/vfio/pci/mlx5/main.c         |  87 +++--
 include/linux/dma-map-ops.h          |  54 ----
 include/linux/dma-mapping.h          |  84 +++++
 include/linux/hmm-dma.h              |  32 ++
 include/linux/hmm.h                  |  16 +
 include/linux/iommu.h                |   4 +
 include/linux/pci-p2pdma.h           |  84 +++++
 include/rdma/ib_umem_odp.h           |  25 +-
 kernel/dma/direct.c                  |  43 ++-
 kernel/dma/mapping.c                 |  20 ++
 mm/hmm.c                             | 229 +++++++++++++-
 21 files changed, 1345 insertions(+), 636 deletions(-)
 create mode 100644 include/linux/hmm-dma.h

2.46.2