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Message-ID: <adb63b87-d8f2-4ae6-90c4-125bde41dc29@samsung.com> Date: Fri, 14 Mar 2025 11:52:58 +0100 From: Marek Szyprowski <m.szyprowski@...sung.com> To: Leon Romanovsky <leon@...nel.org> Cc: Robin Murphy <robin.murphy@....com>, Christoph Hellwig <hch@....de>, Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, Sagi Grimberg <sagi@...mberg.me>, Keith Busch <kbusch@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>, Logan Gunthorpe <logang@...tatee.com>, Yishai Hadas <yishaih@...dia.com>, Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>, Kevin Tian <kevin.tian@...el.com>, Alex Williamson <alex.williamson@...hat.com>, Jérôme Glisse <jglisse@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, linux-block@...r.kernel.org, linux-rdma@...r.kernel.org, iommu@...ts.linux.dev, linux-nvme@...ts.infradead.org, linux-pci@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org, Randy Dunlap <rdunlap@...radead.org> Subject: Re: [PATCH v7 00/17] Provide a new two step DMA mapping API 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@...dia.com> >>>>> >>>>> 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. >> 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@kernel.org/ > [2] https://lore.kernel.org/all/34d44537a65aba6ede215a8ad882aeee028b423a.1730037261.git.leon@kernel.org/ > [3] https://lore.kernel.org/all/383557d0fa1aa393dbab4e1daec94b6cced384ab.1730037261.git.leon@kernel.org/ > > 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@intel.com/ 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. Best regards -- Marek Szyprowski, PhD Samsung R&D Institute Poland
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