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 (orchastrator).
- 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 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 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 pci_p2pdma_map_sg()
instead
of the usual dma_map_sg()
function. Memory mapped in this
way does not need to be unmapped.
The client may also, optionally, make use of
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
pci_p2pdma_distance()
otherwise it may find a memory provider
that’s compatible with all clients using 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
truely 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
pci_alloc_p2pmem()
and pci_free_p2pmem()
to
allocate P2P memory from the provider. pci_p2pmem_alloc_sgl()
and 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.
However, as the memory is not cache coherent, if access ever needs to
be protected by a spinlock then mmiowb()
must be used before
unlocking the lock. (See ACQUIRES VS I/O ACCESSES in
Documentation/memory-barriers.txt)
P2P DMA Support Library¶
-
int
pci_p2pdma_add_resource
(struct pci_dev * pdev, int bar, size_t size, u64 offset)¶ add memory for use as p2p memory
Parameters
struct pci_dev * pdev
- the device to add the memory to
int bar
- PCI BAR to add
size_t size
- size of the memory to add, may be zero to use the whole BAR
u64 offset
- offset into the PCI BAR
Description
The memory will be given ZONE_DEVICE struct pages so that it may be used with any DMA request.
-
int
pci_p2pdma_distance_many
(struct pci_dev * provider, struct device ** clients, int num_clients, bool verbose)¶ Determive the cumulative distance between a p2pdma provider and the clients in use.
Parameters
struct pci_dev * provider
- p2pdma provider to check against the client list
struct device ** clients
- array of devices to check (NULL-terminated)
int num_clients
- number of clients in the array
bool verbose
- if true, print warnings for devices when we return -1
Description
Returns -1 if any of the clients are not compatible (behind the same root port as the provider), otherwise returns a positive number where a lower number is the preferrable choice. (If there’s one client that’s the same as the provider it will return 0, which is best choice).
For now, “compatible” means the provider and the clients are all behind the same PCI root port. This cuts out cases that may work but is safest for the user. Future work can expand this to white-list root complexes that can safely forward between each ports.
-
bool
pci_has_p2pmem
(struct pci_dev * pdev)¶ check if a given PCI device has published any p2pmem
Parameters
struct pci_dev * pdev
- PCI device to check
-
struct pci_dev *
pci_p2pmem_find_many
(struct device ** clients, int num_clients)¶ find a peer-to-peer DMA memory device compatible with the specified list of clients and shortest distance (as determined by
pci_p2pmem_dma()
)
Parameters
struct device ** clients
- array of devices to check (NULL-terminated)
int num_clients
- number of client devices in the list
Description
If multiple devices are behind the same switch, the one “closest” to the client devices in use will be chosen first. (So if one of the providers are the same as one of the clients, that provider will be used ahead of any other providers that are unrelated). If multiple providers are an equal distance away, one will be chosen at random.
Returns a pointer to the PCI device with a reference taken (use pci_dev_put to return the reference) or NULL if no compatible device is found. The found provider will also be assigned to the client list.
-
void *
pci_alloc_p2pmem
(struct pci_dev * pdev, size_t size)¶ allocate peer-to-peer DMA memory
Parameters
struct pci_dev * pdev
- the device to allocate memory from
size_t size
- number of bytes to allocate
Description
Returns the allocated memory or NULL on error.
-
void
pci_free_p2pmem
(struct pci_dev * pdev, void * addr, size_t size)¶ free peer-to-peer DMA memory
Parameters
struct pci_dev * pdev
- the device the memory was allocated from
void * addr
- address of the memory that was allocated
size_t size
- number of bytes that was allocated
-
pci_bus_addr_t
pci_p2pmem_virt_to_bus
(struct pci_dev * pdev, void * addr)¶ return the PCI bus address for a given virtual address obtained with
pci_alloc_p2pmem()
Parameters
struct pci_dev * pdev
- the device the memory was allocated from
void * addr
- address of the memory that was allocated
-
struct scatterlist *
pci_p2pmem_alloc_sgl
(struct pci_dev * pdev, unsigned int * nents, u32 length)¶ allocate peer-to-peer DMA memory in a scatterlist
Parameters
struct pci_dev * pdev
- the device to allocate memory from
unsigned int * nents
- the number of SG entries in the list
u32 length
- number of bytes to allocate
Description
Returns 0 on success
-
void
pci_p2pmem_free_sgl
(struct pci_dev * pdev, struct scatterlist * sgl)¶ free a scatterlist allocated by
pci_p2pmem_alloc_sgl()
Parameters
struct pci_dev * pdev
- the device to allocate memory from
struct scatterlist * sgl
- the allocated scatterlist
-
void
pci_p2pmem_publish
(struct pci_dev * pdev, bool publish)¶ publish the peer-to-peer DMA memory for use by other devices with
pci_p2pmem_find()
Parameters
struct pci_dev * pdev
- the device with peer-to-peer DMA memory to publish
bool publish
- set to true to publish the memory, false to unpublish it
Description
Published memory can be used by other PCI device drivers for peer-2-peer DMA operations. Non-published memory is reserved for exlusive use of the device driver that registers the peer-to-peer memory.
-
int
pci_p2pdma_map_sg
(struct device * dev, struct scatterlist * sg, int nents, enum dma_data_direction dir)¶ map a PCI peer-to-peer scatterlist for DMA
Parameters
struct device * dev
- device doing the DMA request
struct scatterlist * sg
- scatter list to map
int nents
- elements in the scatterlist
enum dma_data_direction dir
- DMA direction
Description
Scatterlists mapped with this function should not be unmapped in any way.
Returns the number of SG entries mapped or 0 on error.
-
int
pci_p2pdma_enable_store
(const char * page, struct pci_dev ** p2p_dev, bool * use_p2pdma)¶ parse a configfs/sysfs attribute store to enable p2pdma
Parameters
const char * page
- contents of the value to be stored
struct pci_dev ** p2p_dev
- returns the PCI device that was selected to be used (if one was specified in the stored value)
bool * use_p2pdma
- returns whether to enable p2pdma or not
Description
Parses an attribute value to decide whether to enable p2pdma.
The value can select a PCI device (using it’s full BDF device
name) or a boolean (in any format strtobool()
accepts). A false
value disables p2pdma, a true value expects the caller
to automatically find a compatible device and specifying a PCI device
expects the caller to use the specific provider.
pci_p2pdma_enable_show()
should be used as the show operation for
the attribute.
Returns 0 on success
-
ssize_t
pci_p2pdma_enable_show
(char * page, struct pci_dev * p2p_dev, bool use_p2pdma)¶ show a configfs/sysfs attribute indicating whether p2pdma is enabled
Parameters
char * page
- contents of the stored value
struct pci_dev * p2p_dev
- the selected p2p device (NULL if no device is selected)
bool use_p2pdma
- whether p2pdme has been enabled
Description
Attributes that use pci_p2pdma_enable_store()
should use this function
to show the value of the attribute.
Returns 0 on success