NAME

       ibv_alloc_null_mr - allocate a null memory region (MR)

SYNOPSIS

              #include <infiniband/verbs.h>

              struct ibv_mr *ibv_alloc_null_mr(struct ibv_pd *pd);

DESCRIPTION

       ibv_alloc_null_mr()  allocates  a  null  memory  region  (MR)  that is associated with the
       protection domain pd.

       A null MR discards all data written to it, and always returns  0  on  read.   It  has  the
       maximum length and only the lkey is valid, the MR is not exposed as an rkey.

       A  device  should  implement  the null MR in a way that bypasses PCI transfers, internally
       discarding or sourcing 0 data.  This provides a way to avoid PCI bus transfers by using  a
       scatter/gather  list in commands if applications do not intend to access the data, or need
       data to be 0 filled.

       Specifically  upon  ibv_post_send()  the  device  skips   PCI   read   cycles   and   upon
       ibv_post_recv() the device skips PCI write cycles which finally improves performance.

       ibv_dereg_mr()  deregisters  the MR.  The use of ibv_rereg_mr() or ibv_bind_mw() with this
       MR is invalid.

RETURN VALUE

       ibv_alloc_null_mr() returns a pointer to the allocated MR, or NULL if the request fails.

SEE ALSO

       ibv_reg_mr(3), ibv_dereg_mr(3),

AUTHOR

       Yonatan Cohen <yonatanc@mellanox.com>