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| -rw-r--r-- | Documentation/driver-api/pci/p2pdma.rst | 141 |
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diff --git a/Documentation/driver-api/pci/p2pdma.rst b/Documentation/driver-api/pci/p2pdma.rst new file mode 100644 index 000000000..44deb52be --- /dev/null +++ b/Documentation/driver-api/pci/p2pdma.rst @@ -0,0 +1,141 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================ +PCI Peer-to-Peer DMA Support +============================ + +The PCI bus has pretty decent support for performing DMA transfers +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. + + +Driver Writer's Guide +===================== + +In a given P2P implementation there may be three or more different +types of kernel drivers in play: + +* Provider - A driver which provides or publishes P2P resources like + memory or doorbell registers to other drivers. +* Client - A driver which makes use of a resource by setting up a + DMA transaction to or from it. +* Orchestrator - A driver which orchestrates the flow of data between + clients and providers. + +In many cases there could be overlap between these three types (i.e., +it may be typical for a driver to be both a provider and a client). + +For example, in the NVMe Target Copy Offload implementation: + +* The NVMe PCI driver is both a client, provider and orchestrator + in that it exposes any CMB (Controller Memory Buffer) as a P2P memory + resource (provider), it accepts P2P memory pages as buffers in requests + to be used directly (client) and it can also make use of the CMB as + submission queue entries (orchestrator). +* The RDMA driver is a client in this arrangement so that an RNIC + can DMA directly to the memory exposed by the NVMe device. +* The NVMe Target driver (nvmet) can orchestrate the data from the RNIC + to the P2P memory (CMB) and then to the NVMe device (and vice versa). + +This is currently the only arrangement supported by the kernel but +one could imagine slight tweaks to this that would allow for the same +functionality. For example, if a specific RNIC added a BAR with some +memory behind it, its driver could add support as a P2P provider and +then the NVMe Target could use the RNIC's memory instead of the CMB +in cases where the NVMe cards in use do not have CMB support. + + +Provider Drivers +---------------- + +A provider simply needs to register a BAR (or a portion of a BAR) +as a P2P DMA resource using :c:func:`pci_p2pdma_add_resource()`. +This will register struct pages for all the specified memory. + +After that it may optionally publish all of its resources as +P2P memory using :c:func:`pci_p2pmem_publish()`. This will allow +any orchestrator drivers to find and use the memory. When marked in +this way, the resource must be regular memory with no side effects. + +For the time being this is fairly rudimentary in that all resources +are typically going to be P2P memory. Future work will likely expand +this to include other types of resources like doorbells. + + +Client Drivers +-------------- + +A client driver typically only has to conditionally change its DMA map +routine to use the mapping function :c:func:`pci_p2pdma_map_sg()` instead +of the usual :c:func:`dma_map_sg()` function. Memory mapped in this +way does not need to be unmapped. + +The client may also, optionally, make use of +:c:func:`is_pci_p2pdma_page()` to determine when to use the P2P mapping +functions and when to use the regular mapping functions. In some +situations, it may be more appropriate to use a flag to indicate a +given request is P2P memory and map appropriately. It is important to +ensure that struct pages that back P2P memory stay out of code that +does not have support for them as other code may treat the pages as +regular memory which may not be appropriate. + + +Orchestrator Drivers +-------------------- + +The first task an orchestrator driver must do is compile a list of +all client devices that will be involved in a given transaction. For +example, the NVMe Target driver creates a list including the namespace +block device and the RNIC in use. If the orchestrator has access to +a specific P2P provider to use it may check compatibility using +:c:func:`pci_p2pdma_distance()` otherwise it may find a memory provider +that's compatible with all clients using :c:func:`pci_p2pmem_find()`. +If more than one provider is supported, the one nearest to all the clients will +be chosen first. If more than one provider is an equal distance away, the +one returned will be chosen at random (it is not an arbitrary but +truly random). This function returns the PCI device to use for the provider +with a reference taken and therefore when it's no longer needed it should be +returned with pci_dev_put(). + +Once a provider is selected, the orchestrator can then use +:c:func:`pci_alloc_p2pmem()` and :c:func:`pci_free_p2pmem()` to +allocate P2P memory from the provider. :c:func:`pci_p2pmem_alloc_sgl()` +and :c:func:`pci_p2pmem_free_sgl()` are convenience functions for +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. + +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. + + +P2P DMA Support Library +======================= + +.. kernel-doc:: drivers/pci/p2pdma.c + :export: |