int ibv_query_qp(struct ibv_qp *qp, struct ibv_qp_attr *attr,
                 enum ibv_qp_attr_mask attr_mask,
                 struct ibv_qp_init_attr *init_attr);

struct ibv_qp_attr {
	enum ibv_qp_state	qp_state;
	enum ibv_qp_state	cur_qp_state;
	enum ibv_mtu		path_mtu;
	enum ibv_mig_state	path_mig_state;
	uint32_t		qkey;
	uint32_t		rq_psn;
	uint32_t		sq_psn;
	uint32_t		dest_qp_num;
	int			qp_access_flags;
	struct ibv_qp_cap	cap;
	struct ibv_ah_attr	ah_attr;
	struct ibv_ah_attr	alt_ah_attr;
	uint16_t		pkey_index;
	uint16_t		alt_pkey_index;
	uint8_t			en_sqd_async_notify;
	uint8_t			sq_draining;
	uint8_t			max_rd_atomic;
	uint8_t			max_dest_rd_atomic;
	uint8_t			min_rnr_timer;
	uint8_t			port_num;
	uint8_t			timeout;
	uint8_t			retry_cnt;
	uint8_t			rnr_retry;
	uint8_t			alt_port_num;
	uint8_t			alt_timeout;
};

Assume that this is the current QP state. This is useful if it is known to the application that the QP state is different from the assumed state by the low-level driver. It can be one of the enumerated values as qp_state. Not relevant for ibv_query_qp()

假设这是当前的QP状态。如果应用程序知道QP状态与底层驱动程序假定的状态不同，这将很有用。它可以是qp_state枚举值之一。与ibv_query_qp（）不相关

路径MTU（path MTU）（最大传输单位），即可以在路径中传输的数据包的最大有效负载大小。它可以是下列枚举值之一：

• IBV_MTU_256 - 256 bytes
• IBV_MTU_512 - 512 bytes
• IBV_MTU_1024 - 1024 bytes
• IBV_MTU_2048 - 2048 bytes
• IBV_MTU_4096 - 4096 bytes

对于UC和RC QP，在需要时，RDMA设备将自动将消息分段为该大小的数据包。

The state of the QP's path migration state machine if supported by the device (IBV_DEVICE_AUTO_PATH_MIG is set in dev_cap.device_cap_flags). It can be one of the following enumerated values:

如果设备支持QP的路径迁移状态机的状态（IBV_DEVICE_AUTO_PATH_MIG在dev_cap.device_cap_flags中设置）。它可以是下列枚举值之一：

• IBV_MIG_MIGRATED - 迁移了路径迁移的状态机，即完成了迁移的初始状态The state machine of path migration is Migrated, i.e. initial state of migration was done
• IBV_MIG_REARM - The state machine of path migration is Rearm, i.e. attempt to try to coordinate the remote RC QP to move both local and remote QPs to Armed state
• IBV_MIG_ARMED - The state machine of path migration is Armed, i.e. both local and remote QPs are ready to perform a path migration

• IBV_ACCESS_REMOTE_WRITE - Allow incoming RDMA Writes on this QP
• IBV_ACCESS_REMOTE_READ - Allow incoming RDMA Reads on this QP
• IBV_ACCESS_REMOTE_ATOMIC - Allow incoming Atomic operations on this QP

Minimum RNR NAK Timer Field Value. When an incoming message to this QP should consume a Work Request from the Receive Queue, but not Work Request is outstanding on that Queue, the QP will send an RNR NAK packet to the initiator. It does not affect RNR NAKs sent for other reasons. The value can be one of the following numeric values since those values aren’t enumerated:

最小RNR NAK计时器字段值。当传入消息到达此QP，将消费接收队列中的一个WR (Work Request)。但该队列上未完成工作请求时，QP将向发起方发送RNR NAK数据包。它不会影响由于其他原因发送的RNR NAK。该值可以是以下数字值之一（这些值未被枚举）：

仅与RC QP相关。

• 0 - 655.36 milliseconds delay
• 1 - 0.01 milliseconds delay
• 2 - 0.02 milliseconds delay
• 3 - 0.03 milliseconds delay
• 4 - 0.04 milliseconds delay
• 5 - 0.06 milliseconds delay
• 6 - 2.56 milliseconds delay
• 7 - 3.84 milliseconds delay
• 8 - 5.12 milliseconds delay
• 9 - 7.68 milliseconds delay
• 10 - 10.24 milliseconds delay
• 11 - 15.36 milliseconds delay
• 12 - 0.08 milliseconds delay
• 13 - 0.12 milliseconds delay
• 14 - 0.16 milliseconds delay
• 15 - 0.24 milliseconds delay
• 16 - 0.32 milliseconds delay
• 17 - 0.48 milliseconds delay
• 18 - 0.64 milliseconds delay
• 19 - 0.96 milliseconds delay
• 20 - 1.28 milliseconds delay
• 21 - 1.92 milliseconds delay
• 22 - 20.48 milliseconds delay
• 23 - 30.72 milliseconds delay
• 24 - 40.96 milliseconds delay
• 25 - 61.44 milliseconds delay
• 26 - 81.92 milliseconds delay
• 27 - 122.88 milliseconds delay
• 28 - 163.84 milliseconds delay
• 29 - 245.76 milliseconds delay
• 30 - 327.68 milliseconds delay
• 31 - 491.52 milliseconds delay

• 0 - infinite
• 1 - 8.192 usec (0.000008 sec)
• 2 - 16.384 usec (0.000016 sec)
• 3 - 32.768 usec (0.000032 sec)
• 4 - 65.536 usec (0.000065 sec)
• 5 - 131.072 usec (0.000131 sec)
• 6 - 262.144 usec (0.000262 sec)
• 7 - 524.288 usec (0.000524 sec)
• 8 - 1048.576 usec (0.00104 sec)
• 9 - 2097.152 usec (0.00209 sec)
• 10 - 4194.304 usec (0.00419 sec)
• 11 - 8388.608 usec (0.00838 sec)
• 12 - 16777.22 usec (0.01677 sec)
• 13 - 33554.43 usec (0.0335 sec)
• 14 - 67108.86 usec (0.0671 sec)
• 15 - 134217.7 usec (0.134 sec)
• 16 - 268435.5 usec (0.268 sec)
• 17 - 536870.9 usec (0.536 sec)
• 18 - 1073742 usec (1.07 sec)
• 19 - 2147484 usec (2.14 sec)
• 20 - 4294967 usec (4.29 sec)
• 21 - 8589935 usec (8.58 sec)
• 22 - 17179869 usec (17.1 sec)
• 23 - 34359738 usec (34.3 sec)
• 24 - 68719477 usec (68.7 sec)
• 25 - 137000000 usec (137 sec)
• 26 - 275000000 usec (275 sec)
• 27 - 550000000 usec (550 sec)
• 28 - 1100000000 usec (1100 sec)
• 29 - 2200000000 usec (2200 sec)
• 30 - 4400000000 usec (4400 sec)
• 31 - 8800000000 usec (8800 sec)

Relevant only to RC QPs

struct ibv_qp_init_attr {
	void		       *qp_context;
	struct ibv_cq	       *send_cq;
	struct ibv_cq	       *recv_cq;
	struct ibv_srq	       *srq;
	struct ibv_qp_cap	cap;
	enum ibv_qp_type	qp_type;
	int			sq_sig_all;
};

struct ibv_qp_cap {
	uint32_t		max_send_wr;
	uint32_t		max_recv_wr;
	uint32_t		max_send_sge;
	uint32_t		max_recv_sge;
	uint32_t		max_inline_data;
};

struct ibv_ah_attr {
	struct ibv_global_route	grh;
	uint16_t		dlid;
	uint8_t			sl;
	uint8_t			src_path_bits;
	uint8_t			static_rate;
	uint8_t			is_global;
	uint8_t			port_num;
};

struct ibv_global_route {
	union ibv_gid		dgid;
	uint32_t		flow_label;
	uint8_t			sgid_index;
	uint8_t			hop_limit;
	uint8_t			traffic_class;
};

struct ibv_qp *qp;
struct ibv_qp_attr attr;
struct ibv_qp_init_attr init_attr;
 
if (ibv_query_qp(qp, &attr,
		  IBV_QP_STATE, &init_attr)) {
	fprintf(stderr, "Failed to query QP state\n");
	return -1;
}