[PATCH v8 05/11] PCI/P2PDMA: Document DMABUF model

Leon Romanovsky
2025-11-11T09:58:32Z

From: Jason Gunthorpe <jgg@nvidia.com>

Reflect latest changes in p2p implementation to support DMABUF lifecycle.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
Documentation/driver-api/pci/p2pdma.rst | 95 +++++++++++++++++++++++++--------
1 file changed, 72 insertions(+), 23 deletions(-)

diff --git a/Documentation/driver-api/pci/p2pdma.rst b/Documentation/driver-api/pci/p2pdma.rst
index d0b241628cf1..77e310596955 100644
--- a/Documentation/driver-api/pci/p2pdma.rst
+++ b/Documentation/driver-api/pci/p2pdma.rst
@@ -9,22 +9,47 @@ between two devices on the bus. This type of transaction is henceforth
called Peer-to-Peer (or P2P). However, there are a number of issues that
make P2P transactions tricky to do in a perfectly safe way.

-One of the biggest issues is that PCI doesn't require forwarding
-transactions between hierarchy domains, and in PCIe, each Root Port
-defines a separate hierarchy domain. To make things worse, there is no
-simple way to determine if a given Root Complex supports this or not.
-(See PCIe r4.0, sec 1.3.1). Therefore, as of this writing, the kernel
-only supports doing P2P when the endpoints involved are all behind the
-same PCI bridge, as such devices are all in the same PCI hierarchy
-domain, and the spec guarantees that all transactions within the
-hierarchy will be routable, but it does not require routing
-between hierarchies.
-
-The second issue is that to make use of existing interfaces in Linux,
-memory that is used for P2P transactions needs to be backed by struct
-pages. However, PCI BARs are not typically cache coherent so there are
-a few corner case gotchas with these pages so developers need to
-be careful about what they do with them.
+For PCIe the routing of Transaction Layer Packets (TLPs) is well-defined up
+until they reach a host bridge or root port. If the path includes PCIe switches
+then based on the ACS settings the transaction can route entirely within
+the PCIe hierarchy and never reach the root port. The kernel will evaluate
+the PCIe topology and always permit P2P in these well-defined cases.
+
+However, if the P2P transaction reaches the host bridge then it might have to
+hairpin back out the same root port, be routed inside the CPU SOC to another
+PCIe root port, or routed internally to the SOC.
+
+As this is not well-defined or well-supported in real HW the kernel defaults to
+blocking such routing. There is an allow list to allow detecting known-good HW,
+in which case P2P between any two PCIe devices will be permitted.
+
+Since P2P inherently is doing transactions between two devices it requires two
+drivers to be co-operating inside the kernel. The providing driver has to convey
+its MMIO to the consuming driver. To meet the driver model lifecycle rules the
+MMIO must have all DMA mapping removed, all CPU accesses prevented, all page
+table mappings undone before the providing driver completes remove().
+
+This requires the providing and consuming driver to actively work together to
+guarantee that the consuming driver has stopped using the MMIO during a removal
+cycle. This is done by either a synchronous invalidation shutdown or waiting
+for all usage refcounts to reach zero.
+
+At the lowest level the P2P subsystem offers a naked struct p2p_provider that
+delegates lifecycle management to the providing driver. It is expected that
+drivers using this option will wrap their MMIO memory in DMABUF and use DMABUF
+to provide an invalidation shutdown. These MMIO pages have no struct page, and
+if used with mmap() must create special PTEs. As such there are very few
+kernel uAPIs that can accept pointers to them; in particular they cannot be used
+with read()/write(), including O_DIRECT.
+
+Building on this, the subsystem offers a layer to wrap the MMIO in a ZONE_DEVICE
+pgmap of MEMORY_DEVICE_PCI_P2PDMA to create struct pages. The lifecycle of
+pgmap ensures that when the pgmap is destroyed all other drivers have stopped
+using the MMIO. This option works with O_DIRECT flows, in some cases, if the
+underlying subsystem supports handling MEMORY_DEVICE_PCI_P2PDMA through
+FOLL_PCI_P2PDMA. The use of FOLL_LONGTERM is prevented. As this relies on pgmap
+it also relies on architecture support along with alignment and minimum size
+limitations.

Driver Writer's Guide
@@ -114,14 +139,38 @@ allocating scatter-gather lists with P2P memory.
Struct Page Caveats
-------------------

-Driver writers should be very careful about not passing these special
-struct pages to code that isn't prepared for it. At this time, the kernel
-interfaces do not have any checks for ensuring this. This obviously
-precludes passing these pages to userspace.
+While the MEMORY_DEVICE_PCI_P2PDMA pages can be installed in VMAs,
+pin_user_pages() and related will not return them unless FOLL_PCI_P2PDMA is set.

-P2P memory is also technically IO memory but should never have any side
-effects behind it. Thus, the order of loads and stores should not be important
-and ioreadX(), iowriteX() and friends should not be necessary.
+The MEMORY_DEVICE_PCI_P2PDMA pages require care to support in the kernel. The
+KVA is still MMIO and must still be accessed through the normal
+readX()/writeX()/etc helpers. Direct CPU access (e.g. memcpy) is forbidden, just
+like any other MMIO mapping. While this will actually work on some
+architectures, others will experience corruption or just crash in the kernel.
+Supporting FOLL_PCI_P2PDMA in a subsystem requires scrubbing it to ensure no CPU
+access happens.
+
+
+Usage With DMABUF
+=================
+
+DMABUF provides an alternative to the above struct page-based
+client/provider/orchestrator system. In this mode the exporting driver will wrap
+some of its MMIO in a DMABUF and give the DMABUF FD to userspace.
+
+Userspace can then pass the FD to an importing driver which will ask the
+exporting driver to map it.
+
+In this case the initiator and target pci_devices are known and the P2P subsystem
+is used to determine the mapping type. The phys_addr_t-based DMA API is used to
+establish the dma_addr_t.
+
+Lifecycle is controlled by DMABUF move_notify(). When the exporting driver wants
+to remove() it must deliver an invalidation shutdown to all DMABUF importing
+drivers through move_notify() and synchronously DMA unmap all the MMIO.
+
+No importing driver can continue to have a DMA map to the MMIO after the
+exporting driver has destroyed its p2p_provider.

P2P DMA Support Library

--
2.51.1