On Fri, Mar 14, 2025 at 11:52:58AM +0100, Marek Szyprowski wrote: > On 12.03.2025 20:32, Leon Romanovsky wrote: > > On Wed, Mar 12, 2025 at 10:28:32AM +0100, Marek Szyprowski wrote: > >> Hi Robin > >> > >> On 28.02.2025 20:54, Robin Murphy wrote: > >>> On 20/02/2025 12:48 pm, Leon Romanovsky wrote: > >>>> On Wed, Feb 05, 2025 at 04:40:20PM +0200, Leon Romanovsky wrote: > >>>>> From: Leon Romanovsky <leonro@xxxxxxxxxx> > >>>>> > >>>>> Changelog: > >>>>> v7: > >>>>> * Rebased to v6.14-rc1 > >>>> <...> > >>>> > >>>>> 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 (11): > >>>>> iommu: add kernel-doc for iommu_unmap and iommu_unmap_fast > >>>>> dma-mapping: 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: Enable the 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 | 468 > >>>>> +++++++++++++++++++++++---- > >>>>> drivers/iommu/iommu.c | 84 ++--- > >>>>> drivers/pci/p2pdma.c | 38 +-- > >>>>> drivers/vfio/pci/mlx5/cmd.c | 375 +++++++++++---------- > >>>>> drivers/vfio/pci/mlx5/cmd.h | 35 +- > >>>>> drivers/vfio/pci/mlx5/main.c | 87 +++-- > >>>>> include/linux/dma-map-ops.h | 54 ---- > >>>>> include/linux/dma-mapping.h | 85 +++++ > >>>>> include/linux/hmm-dma.h | 33 ++ > >>>>> include/linux/hmm.h | 21 ++ > >>>>> include/linux/iommu.h | 4 + > >>>>> include/linux/pci-p2pdma.h | 84 +++++ > >>>>> include/rdma/ib_umem_odp.h | 25 +- > >>>>> kernel/dma/direct.c | 44 +-- > >>>>> kernel/dma/mapping.c | 18 ++ > >>>>> mm/hmm.c | 264 +++++++++++++-- > >>>>> 21 files changed, 1435 insertions(+), 693 deletions(-) > >>>>> create mode 100644 include/linux/hmm-dma.h > >>>> Kind reminder. > > <...> > > > >> Removing the need for scatterlists was advertised as the main goal of > >> this new API, but it looks that similar effects can be achieved with > >> just iterating over the pages and calling page-based DMA API directly. > > Such iteration can't be enough because P2P pages don't have struct pages, > > so you can't use reliably and efficiently dma_map_page_attrs() call. > > > > The only way to do so is to use dma_map_sg_attrs(), which relies on SG > > (the one that we want to remove) to map P2P pages. > > That's something I don't get yet. How P2P pages can be used with > dma_map_sg_attrs(), but not with dma_map_page_attrs()? Both operate > internally on struct page pointer. Yes, and no. See users of is_pci_p2pdma_page(...) function. In dma_*_sg() APIs, there is a real check and support for p2p. In dma_map_page_attrs() variants, this support is missing (ignored, or error is returned). > > >> Maybe I missed something. I still see some advantages in this DMA API > >> extension, but I would also like to see the clear benefits from > >> introducing it, like perf logs or other benchmark summary. > > We didn't focus yet on performance, however Christoph mentioned in his > > block RFC [1] that even simple conversion should improve performance as > > we are performing one P2P lookup per-bio and not per-SG entry as was > > before [2]. In addition it decreases memory [3] too. > > > > [1] https://lore.kernel.org/all/cover.1730037261.git.leon@xxxxxxxxxx/ > > [2] https://lore.kernel.org/all/34d44537a65aba6ede215a8ad882aeee028b423a.1730037261.git.leon@xxxxxxxxxx/ > > [3] https://lore.kernel.org/all/383557d0fa1aa393dbab4e1daec94b6cced384ab.1730037261.git.leon@xxxxxxxxxx/ > > > > So clear benefits are: > > 1. Ability to use native for subsystem structure, e.g. bio for block, > > umem for RDMA, dmabuf for DRM, e.t.c. It removes current wasteful > > conversions from and to SG in order to work with DMA API. > > 2. Batched request and iotlb sync optimizations (perform only once). > > 3. Avoid very expensive call to pgmap pointer. > > 4. Expose MMIO over VFIO without hacks (PCI BAR doesn't have struct pages). > > See this series for such a hack > > https://lore.kernel.org/all/20250307052248.405803-1-vivek.kasireddy@xxxxxxxxx/ > > I see those benefits and I admit that for typical DMA-with-IOMMU case it > would improve some things. I think that main concern from Robin was how > to handle it for the cases without an IOMMU. In such case, we fallback to non-IOMMU flow (old, well-established one). See this HMM patch as an example https://lore.kernel.org/all/a796da065fa8a9cb35d591ce6930400619572dcc.1738765879.git.leonro@xxxxxxxxxx/ +dma_addr_t hmm_dma_map_pfn(struct device *dev, struct hmm_dma_map *map, + size_t idx, + struct pci_p2pdma_map_state *p2pdma_state) ... + if (dma_use_iova(state)) { ... + } else { ... + dma_addr = dma_map_page(dev, page, 0, map->dma_entry_size, + DMA_BIDIRECTIONAL); Thanks > > Best regards > -- > Marek Szyprowski, PhD > Samsung R&D Institute Poland >
