Messages in this thread | | | Subject | Re: [RFC PATCH v2 00/11] Add support to dma_map_sg for P2PDMA | From | Robin Murphy <> | Date | Fri, 12 Mar 2021 15:51:37 +0000 |
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On 2021-03-11 23:31, Logan Gunthorpe wrote: > Hi, > > This is a rework of the first half of my RFC for doing P2PDMA in userspace > with O_DIRECT[1]. > > The largest issue with that series was the gross way of flagging P2PDMA > SGL segments. This RFC proposes a different approach, (suggested by > Dan Williams[2]) which uses the third bit in the page_link field of the > SGL. > > This approach is a lot less hacky but comes at the cost of adding a > CONFIG_64BIT dependency to CONFIG_PCI_P2PDMA and using up the last > scarce bit in the page_link. For our purposes, a 64BIT restriction is > acceptable but it's not clear if this is ok for all usecases hoping > to make use of P2PDMA. > > Matthew Wilcox has already suggested (off-list) that this is the wrong > approach, preferring a new dma mapping operation and an SGL replacement. I > don't disagree that something along those lines would be a better long > term solution, but it involves overcoming a lot of challenges to get > there. Creating a new mapping operation still means adding support to more > than 25 dma_map_ops implementations (many of which are on obscure > architectures) or creating a redundant path to fallback with dma_map_sg() > for every driver that uses the new operation. This RFC is an approach > that doesn't require overcoming these blocks.
I don't really follow that argument - you're only adding support to two implementations with the awkward flag, so why would using a dedicated operation instead be any different? Whatever callers need to do if dma_pci_p2pdma_supported() says no, they could equally do if dma_map_p2p_sg() (or whatever) returns -ENXIO, no?
We don't try to multiplex .map_resource through .map_page, so there doesn't seem to be any good reason to force that complexity on .map_sg either. And having a distinct API from the outset should make it a lot easier to transition to better "list of P2P memory regions" data structures in future without rewriting the whole world. As it is, there are potential benefits in a P2P interface which can define its own behaviour - for instance if threw out the notion of segment merging it could save a load of bother by just maintaining the direct correlation between pages and DMA addresses.
Robin.
> Any alternative ideas or feedback is welcome. > > These patches are based on v5.12-rc2 and a git branch is available here: > > https://github.com/sbates130272/linux-p2pmem/ p2pdma_dma_map_ops_rfc > > A branch with the patches from the previous RFC that add userspace > O_DIRECT support is available at the same URL with the name > "p2pdma_dma_map_ops_rfc+user" (however, none of the issues with those > extra patches from the feedback of the last posting have been fixed). > > Thanks, > > Logan > > [1] https://lore.kernel.org/linux-block/20201106170036.18713-1-logang@deltatee.com/ > [2] https://lore.kernel.org/linux-block/CAPcyv4ifGcrdOtUt8qr7pmFhmecGHqGVre9G0RorGczCGVECQQ@mail.gmail.com/ > > -- > > Logan Gunthorpe (11): > PCI/P2PDMA: Pass gfp_mask flags to upstream_bridge_distance_warn() > PCI/P2PDMA: Avoid pci_get_slot() which sleeps > PCI/P2PDMA: Attempt to set map_type if it has not been set > PCI/P2PDMA: Introduce pci_p2pdma_should_map_bus() and > pci_p2pdma_bus_offset() > lib/scatterlist: Add flag for indicating P2PDMA segments in an SGL > dma-direct: Support PCI P2PDMA pages in dma-direct map_sg > dma-mapping: Add flags to dma_map_ops to indicate PCI P2PDMA support > iommu/dma: Support PCI P2PDMA pages in dma-iommu map_sg > block: Add BLK_STS_P2PDMA > nvme-pci: Check DMA ops when indicating support for PCI P2PDMA > nvme-pci: Convert to using dma_map_sg for p2pdma pages > > block/blk-core.c | 2 + > drivers/iommu/dma-iommu.c | 63 +++++++++++++++++++++----- > drivers/nvme/host/core.c | 3 +- > drivers/nvme/host/nvme.h | 2 +- > drivers/nvme/host/pci.c | 38 +++++++--------- > drivers/pci/Kconfig | 2 +- > drivers/pci/p2pdma.c | 89 +++++++++++++++++++++++++++++++------ > include/linux/blk_types.h | 7 +++ > include/linux/dma-map-ops.h | 3 ++ > include/linux/dma-mapping.h | 5 +++ > include/linux/pci-p2pdma.h | 11 +++++ > include/linux/scatterlist.h | 49 ++++++++++++++++++-- > kernel/dma/direct.c | 35 +++++++++++++-- > kernel/dma/mapping.c | 21 +++++++-- > 14 files changed, 271 insertions(+), 59 deletions(-) > > > base-commit: a38fd8748464831584a19438cbb3082b5a2dab15 > -- > 2.20.1 > _______________________________________________ > iommu mailing list > iommu@lists.linux-foundation.org > https://lists.linuxfoundation.org/mailman/listinfo/iommu >
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