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

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Message-ID: <20250322004130.GS126678@ziepe.ca>
Date: Fri, 21 Mar 2025 21:41:30 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: Leon Romanovsky <leon@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Christoph Hellwig <hch@....de>, 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 Fri, Mar 21, 2025 at 12:52:30AM +0100, Marek Szyprowski wrote:
> > Christoph's vision was to make a performance DMA API path that could
> > be used to implement any scatterlist-like data structure very
> > efficiently without having to teach the DMA API about all sorts of
> > scatterlist-like things.
> 
> Thanks for explaining one more motivation behind this patchset!

Sure, no problem.

To close the loop on the bigger picture here..

When you put the parts together:

 1) dma_map_sg is the only API that is both performant and fully
    functional

 2) scatterlist is a horrible leaky design and badly misued all over
    the place. When Logan added SG_DMA_BUS_ADDRESS it became quite
    clear that any significant changes to scatterlist are infeasible,
    or at least we'd break a huge number of untestable legacy drivers
    in the process.

 3) We really want to do full featured performance DMA *without* a
    struct page. This requires changing scatterlist, inventing a new
    scatterlist v2 and DMA map for it, or this idea here of a flexible
    lower level DMA API entry point.

    Matthew has been talking about struct-pageless for a long time now
    from the block/mm direction using folio & memdesc and this is
    meeting his work from the other end of the stack by starting to
    build a way to do DMA on future struct pageless things. This is 
    going to be huge multi-year project but small parts like this need
    to be solved and agreed to make progress.

 4) In the immediate moment we still have problems in VFIO, RDMA, and
    DRM managing P2P transfers because dma_map_resource/page() don't
    properly work, and we don't have struct pages to use
    dma_map_sg(). Hacks around the DMA API have been in the kernel for
    a long time now, we want to see a properly architected solution.

Jason