ibv_poll_cq(3) poll a completion queue (CQ)


#include <infiniband/verbs.h>

int ibv_poll_cq(struct ibv_cq *cq, int num_entries,
struct ibv_wc *wc);


ibv_poll_cq() polls the CQ cq for work completions and returns the first num_entries (or all available completions if the CQ contains fewer than this number) in the array wc. The argument wc is a pointer to an array of ibv_wc structs, as defined in <infiniband/verbs.h>.

struct ibv_wc {
uint64_t                wr_id;          /* ID of the completed Work Request (WR) */
enum ibv_wc_status      status;         /* Status of the operation */
enum ibv_wc_opcode      opcode;         /* Operation type specified in the completed WR */
uint32_t                vendor_err;     /* Vendor error syndrome */
uint32_t                byte_len;       /* Number of bytes transferred */
uint32_t                imm_data;       /* Immediate data (in network byte order) */
uint32_t                qp_num;         /* Local QP number of completed WR */
uint32_t                src_qp;         /* Source QP number (remote QP number) of completed WR (valid only for UD QPs) */
int                     wc_flags;       /* Flags of the completed WR */
uint16_t                pkey_index;     /* P_Key index (valid only for GSI QPs) */
uint16_t                slid;           /* Source LID */
uint8_t                 sl;             /* Service Level */
uint8_t                 dlid_path_bits; /* DLID path bits (not applicable for multicast messages) */

The attribute wc_flags describes the properties of the work completion. It is either 0 or the bitwise OR of one or more of the following flags:

IBV_WC_GRH GRH is present (valid only for UD QPs)
IBV_WC_WITH_IMM Immediate data value is valid
IBV_WC_IP_CSUM_OK TCP/UDP checksum over IPv4 and IPv4 header checksum are
verified. Valid only when device_cap_flags in device_attr indicates current QP is supported by checksum offload.

Not all wc attributes are always valid. If the completion status is other than IBV_WC_SUCCESS, only the following attributes are valid: wr_id, status, qp_num, and vendor_err.


On success, ibv_poll_cq() returns a non-negative value equal to the number of completions found. On failure, a negative value is returned.


Each polled completion is removed from the CQ and cannot be returned to it.

The user should consume work completions at a rate that prevents CQ overrun from occurrence. In case of a CQ overrun, the async event IBV_EVENT_CQ_ERR will be triggered, and the CQ cannot be used.


Dotan Barak <[email protected]>