fi_mr - Memory region operations

fi_mr_reg / fi_mr_regv / fi_mr_regattr
Register local memory buffers for direct fabric access
Deregister registered memory buffers.
Return a local descriptor associated with a registered memory region
Return the remote key needed to access a registered memory region
Associate a registered memory region with a completion counter.


#include <rdma/fi_domain.h>

int fi_mr_reg(struct fid_domain *domain, const void *buf, size_t len,
    uint64_t access, uint64_t offset, uint64_t requested_key,
    uint64_t flags, struct fid_mr **mr, void *context);

int fi_mr_regv(struct fid_domain *domain, const struct iovec * iov,
    size_t count, uint64_t access, uint64_t offset, uint64_t requested_key,
    uint64_t flags, struct fid_mr **mr, void *context);

int fi_mr_regattr(struct fid_domain *domain, const struct fi_mr_attr *attr,
    uint64_t flags, struct fid_mr **mr);

int fi_close(struct fid *mr);

void * fi_mr_desc(struct fid_mr *mr);

uint64_t fi_mr_key(struct fid_mr *mr);

int fi_mr_bind(struct fid_mr *mr, struct fid *bfid, uint64_t flags);


Resource domain
Memory region
Fabric identifier of an associated resource.
User specified context associated with the memory region.
Memory buffer to register with the fabric hardware
Length of memory buffer to register
Vectored memory buffer.
Count of vectored buffer entries.
Memory access permissions associated with registration
Optional specified offset for accessing specified registered buffers. This parameter is reserved for future use and must be 0.
Optional requested remote key associated with registered buffers.
Memory region attributes
Additional flags to apply to the operation.


Registered memory regions associate memory buffers with permissions granted for access by fabric resources. A memory buffer must be registered with a resource domain before it can be used as the target of a remote RMA or atomic data transfer. Additionally, a fabric provider may require that data buffers be registered before being used in local transfers.

A provider may hide local registration requirements from applications by making use of an internal registration cache or similar mechanisms. Such mechanisms, however, may negatively impact performance for some applications, notably those which manage their own network buffers. In order to support as broad range of applications as possible, without unduly affecting their performance, applications that wish to manage their own local memory registrations may do so by using the memory registration calls. Applications may use the FI_LOCAL_MR domain mode bit as a guide.

When the FI_LOCAL_MR mode bit is set, applications must register all data buffers that will be accessed by the local hardware and provide a valid mem_desc parameter into applicable data transfer operations. When FI_LOCAL_MR is zero, applications are not required to register data buffers before using them for local operations (e.g. send and receive data buffers), and the mem_desc parameter into data transfer operations is ignored.

Further behavior of memory registration operations is controlled based on the mr_mode field in the domain attribute.

Basic Memory Registration Mode
If the mr_mode field is set to FI_MR_BASIC, then memory registration operations are set to basic mode. In basic mode, registration occurs on allocated data buffers, and the MR attributes are selected by the provider.

Basic mode uses provider assigned attributes for the registered buffers. The local memory descriptor and remote memory key are selected by the provider. The address used to access a buffer as the target of an RMA or atomic operation is the same as the virtual address of the buffer.

Applications that support the basic registration mode will need to exchange MR parameters with remote peers for RMA and atomic operations. The exchanged data should include both the address of the memory region as well as the MR key.

Scalable Memory Registration Mode
If the mr_mode field is set to FI_MR_SCALABLE, then memory registration operations are set to scalable mode. In scalable mode, registration occurs on memory address ranges, and the MR attributes are selected by the user.

Memory regions registered as the target of RMA and atomic operations are associated with a MR key selected by the application. If local registrations are required (see FI_LOCAL_MR mode), the local descriptor will be the same as the remote key. The resulting memory region will be accessible by remote peers starting at a base address of 0. Because scalable registration mode refers to memory regions, versus data buffers, the address ranges given for a registration request do not need to map to data buffers allocated by the application at the time the registration call is made. That is, an application can register any range of addresses in their virtual address space, whether or not those addresses are backed by physical pages or have been allocated.

The registrations functions – fi_mr_reg, fi_mr_regv, and fi_mr_regattr – are used to register one or more memory regions with fabric resources. The main difference between registration functions are the number and type of parameters that they accept as input. Otherwise, they perform the same general function.

By default, memory registration completes synchronously. I.e. the registration call will not return until the registration has completed. Memory registration can complete asynchronous by binding the resource domain to an event queue using the FI_REG_MR flag. See fi_domain_bind. When memory registration is asynchronous, in order to avoid a race condition between the registration call returning and the corresponding reading of the event from the EQ, the mr output parameter will be written before any event associated with the operation may be read by the application. An asynchronous event will not be generated unless the registration call returns success (0).


The fi_mr_reg call registers the user-specified memory buffer with the resource domain. The buffer is enabled for access by the fabric hardware based on the provided access permissions. Supported access permissions are the bitwise OR of the following:

The memory buffer may be used in outgoing message data transfers. This includes fi_msg and fi_tagged operations.
The memory buffer may be used to receive inbound message transfers. This includes fi_msg and fi_tagged operations.
The memory buffer may be used as the result buffer for RMA read and atomic operations on the initiator side.
The memory buffer may be used as the source buffer for RMA write and atomic operations on the initiator side.
The memory buffer may be used as the source buffer of an RMA read operation on the target side.
The memory buffer may be used as the target buffer of an RMA write or atomic operation.

Registered memory is associated with a local memory descriptor and, optionally, a remote memory key. A memory descriptor is a provider specific identifier associated with registered memory. Memory descriptors often map to hardware specific indices or keys associated with the memory region. Remote memory keys provide limited protection against unwanted access by a remote node. Remote accesses to a memory region must provide the key associated with the registration.

Because MR keys must be provided by a remote process, an application can use the requested_key parameter to indicate that a specific key value be returned. Support for user requested keys is provider specific and is determined by the mr_mode domain attribute.

Remote RMA and atomic operations indicate the location within a registered memory region by specifying an address. The location is referenced by adding the offset to either the base virtual address of the buffer or to 0, depending on the mr_mode.

The offset parameter is reserved for future use and must be 0.

For asynchronous memory registration requests, the result will be reported to the user through an event queue associated with the resource domain. If successful, the allocated memory region structure will be returned to the user through the mr parameter. The mr address must remain valid until the registration operation completes. The context specified with the registration request is returned with the completion event.


The fi_mr_regv call adds support for a scatter-gather list to fi_mr_reg. Multiple memory buffers are registered as a single memory region. Otherwise, the operation is the same.


The fi_mr_regattr call is a more generic, extensible registration call that allows the user to specify the registration request using a struct fi_mr_attr.

struct fi_mr_attr {
	const struct iovec *mr_iov;       /* scatter-gather array */
	size_t             iov_count;     /* # elements in mr_iov */
	uint64_t           access;        /* access permission flags */
	uint64_t           requested_key; /* requested remote key */
	void               *context;      /* user-defined context */


Fi_close is used to release all resources associated with a registering a memory region. Once unregistered, further access to the registered memory is not guaranteed.

When closing the MR, there must be no opened endpoints or counters associated with the MR. If resources are still associated with the MR when attempting to close, the call will return -FI_EBUSY.

fi_mr_desc / fi_mr_key

The local memory descriptor and remote protection key associated with a MR may be obtained by calling fi_mr_desc and fi_mr_key, respectively. The memory registration must have completed successfully before invoking these calls.


The fi_mr_bind function associates a memory region with a counter, for providers that support the generation of completions based on fabric operations. The type of events tracked against the memory region is based on the bitwise OR of the following flags.

Generates an event whenever a remote RMA write or atomic operation modify the memory region.


Flags are reserved for future use and must be 0.


Returns 0 on success. On error, a negative value corresponding to fabric errno is returned.

Fabric errno values are defined in rdma/fi_errno.h.


The requested_key is already in use.
The requested_key is not available. They key may be out of the range supported by the provider, or the provider may not support user-requested memory registration keys.
Returned by fi_mr_bind if the provider does not support reporting events based on access to registered memory regions.
Returned if the specified flags are not supported by the provider.


fi_getinfo(3), fi_endpoint(3), fi_domain(3), fi_rma(3), fi_msg(3), fi_atomic(3)