Re: [PATCH v6 00/16] dma-mapping: migrate to physical address-based API

Leon Romanovsky
2025-09-15T05:48:23Z

On Fri, Sep 12, 2025 at 12:25:38AM +0200, Marek Szyprowski wrote:
> On 09.09.2025 15:27, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@nvidia.com>
> >
> > Changelog:
> > v6:
> > * Based on "dma-debug: don't enforce dma mapping check on noncoherent
> > allocations" patch.
> > * Removed some unused variables from kmsan conversion.
> > * Fixed missed ! in dma check.
> > v5: https://lore.kernel.org/all/cover.1756822782.git.leon@kernel.org
> > * Added Jason's and Keith's Reviewed-by tags
> > * Fixed DMA_ATTR_MMIO check in dma_direct_map_phys
> > * Jason's cleanup suggestions
> > v4: https://lore.kernel.org/all/cover.1755624249.git.leon@kernel.org/
> > * Fixed kbuild error with mismatch in kmsan function declaration due to
> > rebase error.
> > v3: https://lore.kernel.org/all/cover.1755193625.git.leon@kernel.org
> > * Fixed typo in "cacheable" word
> > * Simplified kmsan patch a lot to be simple argument refactoring
> > v2: https://lore.kernel.org/all/cover.1755153054.git.leon@kernel.org
> > * Used commit messages and cover letter from Jason
> > * Moved setting IOMMU_MMIO flag to dma_info_to_prot function
> > * Micro-optimized the code
> > * Rebased code on v6.17-rc1
> > v1: https://lore.kernel.org/all/cover.1754292567.git.leon@kernel.org
> > * Added new DMA_ATTR_MMIO attribute to indicate
> > PCI_P2PDMA_MAP_THRU_HOST_BRIDGE path.
> > * Rewrote dma_map_* functions to use thus new attribute
> > v0: https://lore.kernel.org/all/cover.1750854543.git.leon@kernel.org/
> > ------------------------------------------------------------------------
> >
> > This series refactors the DMA mapping to use physical addresses
> > as the primary interface instead of page+offset parameters. This
> > change aligns the DMA API with the underlying hardware reality where
> > DMA operations work with physical addresses, not page structures.
> >
> > The series maintains export symbol backward compatibility by keeping
> > the old page-based API as wrapper functions around the new physical
> > address-based implementations.
> >
> > This series refactors the DMA mapping API to provide a phys_addr_t
> > based, and struct-page free, external API that can handle all the
> > mapping cases we want in modern systems:
> >
> > - struct page based cacheable DRAM
> > - struct page MEMORY_DEVICE_PCI_P2PDMA PCI peer to peer non-cacheable
> > MMIO
> > - struct page-less PCI peer to peer non-cacheable MMIO
> > - struct page-less "resource" MMIO
> >
> > Overall this gets much closer to Matthew's long term wish for
> > struct-pageless IO to cacheable DRAM. The remaining primary work would
> > be in the mm side to allow kmap_local_pfn()/phys_to_virt() to work on
> > phys_addr_t without a struct page.
> >
> > The general design is to remove struct page usage entirely from the
> > DMA API inner layers. For flows that need to have a KVA for the
> > physical address they can use kmap_local_pfn() or phys_to_virt(). This
> > isolates the struct page requirements to MM code only. Long term all
> > removals of struct page usage are supporting Matthew's memdesc
> > project which seeks to substantially transform how struct page works.
> >
> > Instead make the DMA API internals work on phys_addr_t. Internally
> > there are still dedicated 'page' and 'resource' flows, except they are
> > now distinguished by a new DMA_ATTR_MMIO instead of by callchain. Both
> > flows use the same phys_addr_t.
> >
> > When DMA_ATTR_MMIO is specified things work similar to the existing
> > 'resource' flow. kmap_local_pfn(), phys_to_virt(), phys_to_page(),
> > pfn_valid(), etc are never called on the phys_addr_t. This requires
> > rejecting any configuration that would need swiotlb. CPU cache
> > flushing is not required, and avoided, as ATTR_MMIO also indicates the
> > address have no cacheable mappings. This effectively removes any
> > DMA API side requirement to have struct page when DMA_ATTR_MMIO is
> > used.
> >
> > In the !DMA_ATTR_MMIO mode things work similarly to the 'page' flow,
> > except on the common path of no cache flush, no swiotlb it never
> > touches a struct page. When cache flushing or swiotlb copying
> > kmap_local_pfn()/phys_to_virt() are used to get a KVA for CPU
> > usage. This was already the case on the unmap side, now the map side
> > is symmetric.
> >
> > Callers are adjusted to set DMA_ATTR_MMIO. Existing 'resource' users
> > must set it. The existing struct page based MEMORY_DEVICE_PCI_P2PDMA
> > path must also set it. This corrects some existing bugs where iommu
> > mappings for P2P MMIO were improperly marked IOMMU_CACHE.
> >
> > Since ATTR_MMIO is made to work with all the existing DMA map entry
> > points, particularly dma_iova_link(), this finally allows a way to use
> > the new DMA API to map PCI P2P MMIO without creating struct page. The
> > VFIO DMABUF series demonstrates how this works. This is intended to
> > replace the incorrect driver use of dma_map_resource() on PCI BAR
> > addresses.
> >
> > This series does the core code and modern flows. A followup series
> > will give the same treatment to the legacy dma_ops implementation.
>
> Applied patches 1-13 into dma-mapping-for-next branch. Let's check if it
> works fine in linux-next.

Thanks a lot.

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