Re: [PATCH v7 00/17] Provide a new two step DMA mapping API

[Marek Szyprowski]

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.

>> 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.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland