Subject: Re: [PATCH v4 00/16] dma-mapping: migrate to physical address-based API

From: Leon Romanovsky <[email-masked]>
Date: Thu, 28 Aug 2025 14:57:29 +0300

On Tue, Aug 19, 2025 at 08:36:44PM +0300, Leon Romanovsky wrote:
> Changelog:
> v4:
>  * Fixed kbuild error with mismatch in kmsan function declaration due to
>    rebase error.
> v3: https://lore.kernel.org/all/[email-masked]
>  * Fixed typo in "cacheable" word
>  * Simplified kmsan patch a lot to be simple argument refactoring
> v2: https://lore.kernel.org/all/[email-masked]
>  * 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/[email-masked]
>  * 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/[email-masked]/
> ------------------------------------------------------------------------
> 
> 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 cachable DRAM
>  - struct page MEMORY_DEVICE_PCI_P2PDMA PCI peer to peer non-cachable
>    MMIO
>  - struct page-less PCI peer to peer non-cachable MMIO
>  - struct page-less "resource" MMIO
> 
> Overall this gets much closer to Matthew's long term wish for
> struct-pageless IO to cachable 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 cachable 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.
> 
> Thanks
> 
> Leon Romanovsky (16):
>   dma-mapping: introduce new DMA attribute to indicate MMIO memory
>   iommu/dma: implement DMA_ATTR_MMIO for dma_iova_link().
>   dma-debug: refactor to use physical addresses for page mapping
>   dma-mapping: rename trace_dma_*map_page to trace_dma_*map_phys
>   iommu/dma: rename iommu_dma_*map_page to iommu_dma_*map_phys
>   iommu/dma: extend iommu_dma_*map_phys API to handle MMIO memory
>   dma-mapping: convert dma_direct_*map_page to be phys_addr_t based
>   kmsan: convert kmsan_handle_dma to use physical addresses
>   dma-mapping: handle MMIO flow in dma_map|unmap_page
>   xen: swiotlb: Open code map_resource callback
>   dma-mapping: export new dma_*map_phys() interface
>   mm/hmm: migrate to physical address-based DMA mapping API
>   mm/hmm: properly take MMIO path
>   block-dma: migrate to dma_map_phys instead of map_page
>   block-dma: properly take MMIO path
>   nvme-pci: unmap MMIO pages with appropriate interface
> 
>  Documentation/core-api/dma-api.rst        |   4 +-
>  Documentation/core-api/dma-attributes.rst |  18 ++++
>  arch/powerpc/kernel/dma-iommu.c           |   4 +-
>  block/blk-mq-dma.c                        |  15 ++-
>  drivers/iommu/dma-iommu.c                 |  61 +++++------
>  drivers/nvme/host/pci.c                   |  18 +++-
>  drivers/virtio/virtio_ring.c              |   4 +-
>  drivers/xen/swiotlb-xen.c                 |  21 +++-
>  include/linux/blk-mq-dma.h                |   6 +-
>  include/linux/blk_types.h                 |   2 +
>  include/linux/dma-direct.h                |   2 -
>  include/linux/dma-map-ops.h               |   8 +-
>  include/linux/dma-mapping.h               |  33 ++++++
>  include/linux/iommu-dma.h                 |  11 +-
>  include/linux/kmsan.h                     |   9 +-
>  include/trace/events/dma.h                |   9 +-
>  kernel/dma/debug.c                        |  71 ++++---------
>  kernel/dma/debug.h                        |  37 ++-----
>  kernel/dma/direct.c                       |  22 +---
>  kernel/dma/direct.h                       |  52 ++++++----
>  kernel/dma/mapping.c                      | 117 +++++++++++++---------
>  kernel/dma/ops_helpers.c                  |   6 +-
>  mm/hmm.c                                  |  19 ++--
>  mm/kmsan/hooks.c                          |   5 +-
>  rust/kernel/dma.rs                        |   3 +
>  tools/virtio/linux/kmsan.h                |   2 +-
>  26 files changed, 305 insertions(+), 254 deletions(-)

Marek,

So what are the next steps here? This series is pre-requirement for the
VFIO MMIO patches.

Thanks

> -- 
> 2.50.1