mirror of
https://github.com/torvalds/linux.git
synced 2024-12-22 19:01:37 +00:00
915530396c
The guarantees provided by mmiowb() are now provided implicitly by spin_unlock(), so remove all references to this most confusing of barriers from our Documentation. Good riddance. Acked-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Will Deacon <will.deacon@arm.com>
142 lines
6.2 KiB
ReStructuredText
142 lines
6.2 KiB
ReStructuredText
.. 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:
|