目录
说明
1. RDMA的学习环境搭建
RDMA需要专门的RDMA网卡或者InfiniBand卡才能使用，学习RDMA而又没有这些硬件设备，可以使用一个软件RDMA模拟环境，softiwarp ，
- 这是加载地址：https://github.com/zrlio/softiwarp
- 这是安装教程：http://www.reflectionsofthevoid.com/2011/03/how-to-install-soft-iwarp-on-ubuntu.html
更多的rdmacm实例：，
- https://github.com/tarickb/the-geek-in-the-corner
需要注意的是，这个例子里面缺省用的是IPv6连接，如果希望在IPv4环境下测试，需要先改代码用IPv4地址。
2. RDMA与socket的类比
和Socket连接类似，RDMA连接也分为可靠连接和不可靠连接。然而也不完全相同，Socket的可靠连接就是TCP连接，是流式的；不可靠连接也就是UDP，是消息式的。对于RDMA来说，无论是可靠连接和不可靠连接，都是消息式的。
编程角度看，RDMA代码也分为Server端，Client端，也有bind, listen, connect, accept,等动作，然而细节上仍有不少区别。
rdma_cm API说明：
https://linux.die.net/man/3/rdma_create_id （推荐）
https://www.ibm.com/docs/en/aix/7.2?topic=operations-rdma-listen （内容少）
3. RDMA服务器的代码流程
main（）
{
-
rdma_create_event_channel
这一步是创建一个event channel，event channel是RDMA设备在操作完成后，或者有连接请求等事件发生时，用来通知应用程序的通道。其内部就是一个file descriptor, 因此可以进行poll等操作。 -
rdma_create_id(channel, **id，……)
这一步创建一个rdma_cm_id, 概念上等价与socket编程时的listen socket。 -
rdma_bind_addr(id,addr)
和socket编程一样，也要先绑定一个本地的地址和端口，以进行listen操作。 -
rdma_listen(id,block)
开始侦听客户端的连接请求 -
rdma_get_cm_event
这个调用就是作用在第一步创建的event channel上面，要从event channel中获取一个事件。这是个阻塞调用，只有有事件时才会返回。在一切正常的情况下，函数返回时会得到一个 RDMA_CM_EVENT_CONNECT_REQUEST事件，也就是说，有客户端发起连接了。
在事件的参数里面，会有一个新的rdma_cm_id传入。这点和socket是不同的，socket只有在accept后才有新的socket fd创建。
on_event()
{
on_connect_request()//RDMA_CM_EVENT_CONNECT_REQUEST
{
build_context()
{
-
ibv_alloc_pd
创建一个protection domain。protection domain可以看作是一个内存保护单位，在内存区域和队列直接建立一个关联关系，防止未授权的访问。
-
ibv_create_comp_channel
和之前创建的event channel类似，这也是一个event channel，但只用来报告完成队列里面的事件。当完成队列里有新的任务完成时，就通过这个channel向应用程序报告。
-
ibv_create_cq
创建完成队列，创建时就指定使用第6步的channel。
}//--end build_context()
-
rdma_create_qp
创建一个queue pair, 一个queue pair包括一个发送queue和一个接收queue. 指定使用前面创建的cq作为完成队列。该qp创建时就指定关联到第6步创建的pd上。 -
ibv_reg_mr
注册内存区域。RDMA使用的内存，必须事先进行注册。这个是可以理解的，DMA的内存在边界对齐，能否被swap等方面，都有要求。 -
rdma_accept
至此，做好了全部的准备工作，可以调用accept接受客户端的这个请求了。 –：）长出一口气 ~~ 且慢，
}
//--end on_connect_request()
-
rdma_ack_cm_event
对于每个从event channel得到的事件，都要调用ack函数，否则会产生内存泄漏。这一步的ack是对应第5步的get。每一次get调用，都要有对应的ack调用。 -
rdma_get_cm_event
继续调用rdma_get_cm_event, 一切正常的话我们此时应该得到 RDMA_CM_EVENT_ESTABLISHED 事件，表示连接已经建立起来。不需要做额外的处理，直接rdma_ack_cm_event就行了
}//--end on_event()
终于可以开始进行数据传输了 ==== (如何传输下篇再说)
4. 关闭连接
-
断开连接
当rdma_get_cm_event返回RDMA_CM_EVENT_DISCONNECTED事件时，表示客户端断开了连接，server端要进行对应的清理。此时可以调用rdma_ack_cm_event释放事件资源。然后依次调用下面的函数，释放连接资源，内存资源，队列资源。 -
rdma_disconnect
-
rdma_destroy_qp
-
ibv_dereg_mr
-
rdma_destroy_id
释放同客户端连接的rdma_cm_id -
rdma_destroy_id
释放用于侦听的rdma_cm_id -
rdma_destroy_event_channel
释放 event channel
}
// end main
实例
源码地址- https://github.com/tarickb/the-geek-in-the-corner
Makefile
.PHONY: clean
CFLAGS := -Wall -g
LDLIBS := ${LDLIBS} -lrdmacm -libverbs -lpthreadAPPS := server client
all: ${APPS}
clean:
rm -f ${APPS}
注意：makefile 没有-L 指定lib的路径，所以 -lrdmacm -libverbs -lpthread 对应的库 librdmacm.so libibverbs.so libpthread.so 应放在默认的路径下/usr/lib 或/usr/lib64
server、clicent源码下载：https://download.csdn.net/download/bandaoyu/18630109
服务端server.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <rdma/rdma_cma.h>
#define TEST_NZ(x) do { if ( (x)) die("error: " #x " failed (returned non-zero)." ); } while (0)
#define TEST_Z(x) do { if (!(x)) die("error: " #x " failed (returned zero/null)."); } while (0)
const int BUFFER_SIZE = 1024;
struct context {
struct ibv_context *ctx;
struct ibv_pd *pd;
struct ibv_cq *cq;
struct ibv_comp_channel *comp_channel;
pthread_t cq_poller_thread;
};
struct connection {
struct ibv_qp *qp;
struct ibv_mr *recv_mr;
struct ibv_mr *send_mr;
char *recv_region;
char *send_region;
};
static void die(const char *reason);
static void build_context(struct ibv_context *verbs);
static void build_qp_attr(struct ibv_qp_init_attr *qp_attr);
static void * poll_cq(void *);
static void post_receives(struct connection *conn);
static void register_memory(struct connection *conn);
static void on_completion(struct ibv_wc *wc);
static int on_connect_request(struct rdma_cm_id *id);
static int on_connection(void *context);
static int on_disconnect(struct rdma_cm_id *id);
static int on_event(struct rdma_cm_event *event);
static struct context *s_ctx = NULL;
int main(int argc, char **argv)
{
#if _USE_IPV6
struct sockaddr_in6 addr;
#else
struct sockaddr_in addr;
#endif
struct rdma_cm_event *event = NULL;
struct rdma_cm_id *listener = NULL;
struct rdma_event_channel *ec = NULL;
uint16_t port = 0;
memset(&addr, 0, sizeof(addr));
#if _USE_IPV6
addr.sin6_family = AF_INET6;
#else
addr.sin_family = AF_INET;
#endif
TEST_Z(ec = rdma_create_event_channel());
TEST_NZ(rdma_create_id(ec, &listener, NULL, RDMA_PS_TCP));
TEST_NZ(rdma_bind_addr(listener, (struct sockaddr *)&addr));
TEST_NZ(rdma_listen(listener, 10)); /* backlog=10 is arbitrary */
port = ntohs(rdma_get_src_port(listener)); //rdma_get_src_port 返回listener对应的tcp 端口
printf("listening on port %d.\n", port);
while (rdma_get_cm_event(ec, &event) == 0) {
struct rdma_cm_event event_copy;
memcpy(&event_copy, event, sizeof(*event));
rdma_ack_cm_event(event);
if (on_event(&event_copy))
break;
}
rdma_destroy_id(listener);
rdma_destroy_event_channel(ec);
return 0;
}
void die(const char *reason)
{
fprintf(stderr, "%s\n", reason);
exit(EXIT_FAILURE);
}
void build_context(struct ibv_context *verbs)
{
if (s_ctx) {
if (s_ctx->ctx != verbs)
die("cannot handle events in more than one context.");
return;
}
s_ctx = (struct context *)malloc(sizeof(struct context));
s_ctx->ctx = verbs;
TEST_Z(s_ctx->pd = ibv_alloc_pd(s_ctx->ctx));
TEST_Z(s_ctx->comp_channel = ibv_create_comp_channel(s_ctx->ctx));
TEST_Z(s_ctx->cq = ibv_create_cq(s_ctx->ctx, 10, NULL, s_ctx->comp_channel, 0)); /* cqe=10 is arbitrary */
TEST_NZ(ibv_req_notify_cq(s_ctx->cq, 0)); #完成完成队列与完成通道的关联
TEST_NZ(pthread_create(&s_ctx->cq_poller_thread, NULL, poll_cq, NULL));
}
void build_qp_attr(struct ibv_qp_init_attr *qp_attr)
{
memset(qp_attr, 0, sizeof(*qp_attr));
qp_attr->send_cq = s_ctx->cq;
qp_attr->recv_cq = s_ctx->cq;
qp_attr->qp_type = IBV_QPT_RC;
qp_attr->cap.max_send_wr = 10;
qp_attr->cap.max_recv_wr = 10;
qp_attr->cap.max_send_sge = 1;
qp_attr->cap.max_recv_sge = 1;
}
void * poll_cq(void *ctx)
{
struct ibv_cq *cq;
struct ibv_wc wc;
while (1) {
TEST_NZ(ibv_get_cq_event(s_ctx->comp_channel, &cq, &ctx));
ibv_ack_cq_events(cq, 1);
TEST_NZ(ibv_req_notify_cq(cq, 0));
while (ibv_poll_cq(cq, 1, &wc))
on_completion(&wc);
}
return NULL;
}
void post_receives(struct connection *conn)
{
struct ibv_recv_wr wr, *bad_wr = NULL;
struct ibv_sge sge;
wr.wr_id = (uintptr_t)conn;
wr.next = NULL;
wr.sg_list = &sge;
wr.num_sge = 1;
sge.addr = (uintptr_t)conn->recv_region;
sge.length = BUFFER_SIZE;
sge.lkey = conn->recv_mr->lkey;
TEST_NZ(ibv_post_recv(conn->qp, &wr, &bad_wr));
}
void register_memory(struct connection *conn)
{
conn->send_region = malloc(BUFFER_SIZE);
conn->recv_region = malloc(BUFFER_SIZE);
TEST_Z(conn->send_mr = ibv_reg_mr(
s_ctx->pd,
conn->send_region,
BUFFER_SIZE,
0));
TEST_Z(conn->recv_mr = ibv_reg_mr(
s_ctx->pd,
conn->recv_region,
BUFFER_SIZE,
IBV_ACCESS_LOCAL_WRITE));
}
void on_completion(struct ibv_wc *wc)
{
if (wc->status != IBV_WC_SUCCESS)
die("on_completion: status is not IBV_WC_SUCCESS.");
if (wc->opcode & IBV_WC_RECV) {
struct connection *conn = (struct connection *)(uintptr_t)wc->wr_id;
printf("received message: %s\n", conn->recv_region);
} else if (wc->opcode == IBV_WC_SEND) {
printf("send completed successfully.\n");
}
}
int on_connect_request(struct rdma_cm_id *id)
{
struct ibv_qp_init_attr qp_attr;
struct rdma_conn_param cm_params;
struct connection *conn;
printf("received connection request.\n");
build_context(id->verbs);
build_qp_attr(&qp_attr);
TEST_NZ(rdma_create_qp(id, s_ctx->pd, &qp_attr));
id->context = conn = (struct connection *)malloc(sizeof(struct connection));
conn->qp = id->qp;
register_memory(conn);
post_receives(conn);
memset(&cm_params, 0, sizeof(cm_params));
TEST_NZ(rdma_accept(id, &cm_params));
return 0;
}
int on_connection(void *context)
{
struct connection *conn = (struct connection *)context;
struct ibv_send_wr wr, *bad_wr = NULL;
struct ibv_sge sge;
snprintf(conn->send_region, BUFFER_SIZE, "message from passive/server side with pid %d", getpid());
printf("connected. posting send...\n");
memset(&wr, 0, sizeof(wr));
wr.opcode = IBV_WR_SEND;
wr.sg_list = &sge;
wr.num_sge = 1;
wr.send_flags = IBV_SEND_SIGNALED;
sge.addr = (uintptr_t)conn->send_region;
sge.length = BUFFER_SIZE;
sge.lkey = conn->send_mr->lkey;
TEST_NZ(ibv_post_send(conn->qp, &wr, &bad_wr));
return 0;
}
int on_disconnect(struct rdma_cm_id *id)
{
struct connection *conn = (struct connection *)id->context;
printf("peer disconnected.\n");
rdma_destroy_qp(id);
ibv_dereg_mr(conn->send_mr);
ibv_dereg_mr(conn->recv_mr);
free(conn->send_region);
free(conn->recv_region);
free(conn);
rdma_destroy_id(id);
return 0;
}
int on_event(struct rdma_cm_event *event)
{
int r = 0;
if (event->event == RDMA_CM_EVENT_CONNECT_REQUEST)
r = on_connect_request(event->id);
else if (event->event == RDMA_CM_EVENT_ESTABLISHED)
r = on_connection(event->id->context);
else if (event->event == RDMA_CM_EVENT_DISCONNECTED)
r = on_disconnect(event->id);
else
die("on_event: unknown event.");
return r;
}客户端client.c
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <rdma/rdma_cma.h>
#define TEST_NZ(x) do { if ( (x)) die("error: " #x " failed (returned non-zero)." ); } while (0)
#define TEST_Z(x) do { if (!(x)) die("error: " #x " failed (returned zero/null)."); } while (0)
const int BUFFER_SIZE = 1024;
const int TIMEOUT_IN_MS = 500; /* ms */
struct context {
struct ibv_context *ctx;
struct ibv_pd *pd;
struct ibv_cq *cq;
struct ibv_comp_channel *comp_channel;
pthread_t cq_poller_thread;
};
struct connection {
struct rdma_cm_id *id;
struct ibv_qp *qp;
struct ibv_mr *recv_mr;
struct ibv_mr *send_mr;
char *recv_region;
char *send_region;
int num_completions;
};
static void die(const char *reason);
static void build_context(struct ibv_context *verbs);
static void build_qp_attr(struct ibv_qp_init_attr *qp_attr);
static void * poll_cq(void *);
static void post_receives(struct connection *conn);
static void register_memory(struct connection *conn);
static int on_addr_resolved(struct rdma_cm_id *id);
static void on_completion(struct ibv_wc *wc);
static int on_connection(void *context);
static int on_disconnect(struct rdma_cm_id *id);
static int on_event(struct rdma_cm_event *event);
static int on_route_resolved(struct rdma_cm_id *id);
static struct context *s_ctx = NULL;
int main(int argc, char **argv)
{
struct addrinfo *addr;
struct rdma_cm_event *event = NULL;
struct rdma_cm_id *conn= NULL;
struct rdma_event_channel *ec = NULL;
if (argc != 3)
die("usage: client <server-address> <server-port>");
TEST_NZ(getaddrinfo(argv[1], argv[2], NULL, &addr));
TEST_Z(ec = rdma_create_event_channel());
TEST_NZ(rdma_create_id(ec, &conn, NULL, RDMA_PS_TCP));
TEST_NZ(rdma_resolve_addr(conn, NULL, addr->ai_addr, TIMEOUT_IN_MS));
freeaddrinfo(addr);
while (rdma_get_cm_event(ec, &event) == 0) {
struct rdma_cm_event event_copy;
memcpy(&event_copy, event, sizeof(*event));
rdma_ack_cm_event(event);
if (on_event(&event_copy))
break;
}
rdma_destroy_event_channel(ec);
return 0;
}
void die(const char *reason)
{
fprintf(stderr, "%s\n", reason);
exit(EXIT_FAILURE);
}
void build_context(struct ibv_context *verbs)
{
if (s_ctx) {
if (s_ctx->ctx != verbs)
die("cannot handle events in more than one context.");
return;
}
s_ctx = (struct context *)malloc(sizeof(struct context));
s_ctx->ctx = verbs;
TEST_Z(s_ctx->pd = ibv_alloc_pd(s_ctx->ctx));
TEST_Z(s_ctx->comp_channel = ibv_create_comp_channel(s_ctx->ctx));
TEST_Z(s_ctx->cq = ibv_create_cq(s_ctx->ctx, 10, NULL, s_ctx->comp_channel, 0)); /* cqe=10 is arbitrary */
TEST_NZ(ibv_req_notify_cq(s_ctx->cq, 0));
TEST_NZ(pthread_create(&s_ctx->cq_poller_thread, NULL, poll_cq, NULL));
}
void build_qp_attr(struct ibv_qp_init_attr *qp_attr)
{
memset(qp_attr, 0, sizeof(*qp_attr));
qp_attr->send_cq = s_ctx->cq;
qp_attr->recv_cq = s_ctx->cq;
qp_attr->qp_type = IBV_QPT_RC;
qp_attr->cap.max_send_wr = 10;
qp_attr->cap.max_recv_wr = 10;
qp_attr->cap.max_send_sge = 1;
qp_attr->cap.max_recv_sge = 1;
}
void * poll_cq(void *ctx)
{
struct ibv_cq *cq;
struct ibv_wc wc;
while (1) {
TEST_NZ(ibv_get_cq_event(s_ctx->comp_channel, &cq, &ctx));
ibv_ack_cq_events(cq, 1);
TEST_NZ(ibv_req_notify_cq(cq, 0));
while (ibv_poll_cq(cq, 1, &wc))
on_completion(&wc);
}
return NULL;
}
void post_receives(struct connection *conn)
{
struct ibv_recv_wr wr, *bad_wr = NULL;
struct ibv_sge sge;
wr.wr_id = (uintptr_t)conn;
wr.next = NULL;
wr.sg_list = &sge;
wr.num_sge = 1;
sge.addr = (uintptr_t)conn->recv_region;
sge.length = BUFFER_SIZE;
sge.lkey = conn->recv_mr->lkey;
TEST_NZ(ibv_post_recv(conn->qp, &wr, &bad_wr));
}
void register_memory(struct connection *conn)
{
conn->send_region = malloc(BUFFER_SIZE);
conn->recv_region = malloc(BUFFER_SIZE);
TEST_Z(conn->send_mr = ibv_reg_mr(
s_ctx->pd,
conn->send_region,
BUFFER_SIZE,
0));
TEST_Z(conn->recv_mr = ibv_reg_mr(
s_ctx->pd,
conn->recv_region,
BUFFER_SIZE,
IBV_ACCESS_LOCAL_WRITE));
}
int on_addr_resolved(struct rdma_cm_id *id)
{
struct ibv_qp_init_attr qp_attr;
struct connection *conn;
printf("address resolved.\n");
build_context(id->verbs);
build_qp_attr(&qp_attr);
TEST_NZ(rdma_create_qp(id, s_ctx->pd, &qp_attr));
id->context = conn = (struct connection *)malloc(sizeof(struct connection));
conn->id = id;
conn->qp = id->qp;
conn->num_completions = 0;
register_memory(conn);
post_receives(conn);
TEST_NZ(rdma_resolve_route(id, TIMEOUT_IN_MS));
return 0;
}
void on_completion(struct ibv_wc *wc)
{
struct connection *conn = (struct connection *)(uintptr_t)wc->wr_id;
if (wc->status != IBV_WC_SUCCESS)
die("on_completion: status is not IBV_WC_SUCCESS.");
if (wc->opcode & IBV_WC_RECV)
printf("received message: %s\n", conn->recv_region);
else if (wc->opcode == IBV_WC_SEND)
printf("send completed successfully.\n");
else
die("on_completion: completion isn't a send or a receive.");
if (++conn->num_completions == 2)
rdma_disconnect(conn->id);
}
int on_connection(void *context)
{
struct connection *conn = (struct connection *)context;
struct ibv_send_wr wr, *bad_wr = NULL;
struct ibv_sge sge;
snprintf(conn->send_region, BUFFER_SIZE, "message from active/client side with pid %d", getpid());
printf("connected. posting send...\n");
memset(&wr, 0, sizeof(wr));
wr.wr_id = (uintptr_t)conn;
wr.opcode = IBV_WR_SEND;
wr.sg_list = &sge;
wr.num_sge = 1;
wr.send_flags = IBV_SEND_SIGNALED;
sge.addr = (uintptr_t)conn->send_region;
sge.length = BUFFER_SIZE;
sge.lkey = conn->send_mr->lkey;
TEST_NZ(ibv_post_send(conn->qp, &wr, &bad_wr));
return 0;
}
int on_disconnect(struct rdma_cm_id *id)
{
struct connection *conn = (struct connection *)id->context;
printf("disconnected.\n");
rdma_destroy_qp(id);
ibv_dereg_mr(conn->send_mr);
ibv_dereg_mr(conn->recv_mr);
free(conn->send_region);
free(conn->recv_region);
free(conn);
rdma_destroy_id(id);
return 1; /* exit event loop */
}
int on_event(struct rdma_cm_event *event)
{
int r = 0;
if (event->event == RDMA_CM_EVENT_ADDR_RESOLVED)
r = on_addr_resolved(event->id);
else if (event->event == RDMA_CM_EVENT_ROUTE_RESOLVED)
r = on_route_resolved(event->id);
else if (event->event == RDMA_CM_EVENT_ESTABLISHED)
r = on_connection(event->id->context);
else if (event->event == RDMA_CM_EVENT_DISCONNECTED)
r = on_disconnect(event->id);
else
die("on_event: unknown event.");
return r;
}
int on_route_resolved(struct rdma_cm_id *id)
{
struct rdma_conn_param cm_params;
printf("route resolved.\n");
memset(&cm_params, 0, sizeof(cm_params));
TEST_NZ(rdma_connect(id, &cm_params));
return 0;
}
更多讲解教程
https://thegeekinthecorner.wordpress.com/category/infiniband-verbs-rdma/
https://thegeekinthecorner.wordpress.com/2010/09/28/rdma-read-and-write-with-ib-verbs/
http://www.hpcadvisorycouncil.com/pdf/building-an-rdma-capable-application-with-ib-verbs.pdf
LINUX 编程例子
https://community.mellanox.com/s/topic/0TO50000000g1zhGAA/linux-programming?tabset-dea0d=2
4、rdma_listen()和rdma_request()是否要在一起执行？
不一定。rdma_listen()不是阻塞式方法，只是监听连接请求，不会产生新的连接rdma_cm_id，如果想产生新的rdma_cm_id，需要执行rdma_get_cm_event()获取RDMA_CM_EVENT_CONNECT_REQUEST事件，或者执行rdma_get_request()获取连接请求事件。所以rdma_get_cm_event()和rdma_get_request()的原理是相同的，都是为了获取RDMA_CM_EVENT_CONNECT_REQUEST事件，表示有客户端连接，然后会产生新的rdma_cm_id，也就是连接rdma_cm_id。对于传统的TCP/IP通信，listen阶段只是监听，accept之后才产生新的socket fd，但是RDMA中，在listen阶段即可产生新的rdma_cm_id，更为准确来说，是在listen调用之后，只要有RDMA_CM_EVENT_CONNECT_REQUEST事件产生，就会生成新的rdma_cm_id。尝试过，如果只有listen，没有request，就会报错。可见，listen之后需要有相应的处理机制才可以。
5、为什么要设置IBV_SEND_INLINE？
int send_flags，描述了WR的属性，其值为0或者一个或多个flags的按位异或。
IBV_SEND_FENCE - 为此WR设置围栏指示器。这意味着这个WR的处理将被阻止，直到所有之前发布的RDMA Read和Atomic WR都将被完成。仅对运输服务类型为IBV_QPT_RC的QP有效
IBV_SEND_SIGNALED - 设置此WR的完成通知指示符。这意味着如果QP是使用sq_sig_all = 0创建的，那么当WR的处理结束时，将会产生一个工作完成。如果QP是使用sq_sig_all = 1创建的，则不会对此标志产生任何影响
IBV_SEND_SOLICITED - 为此WR设置请求事件指示器。这意味着，当这个WR中的消息将在远程QP中结束时，将会创建一个请求事件，如果在远程侧，用户正在等待请求事件，它将被唤醒。与仅用于发送和RDMA写入的立即操作码相关
IBV_SEND_INLINE - sg_list中指定的内存缓冲区将内联放置在发送请求中。这意味着低级驱动程序（即CPU）将读取数据而不是RDMA设备。这意味着L_Key将不会被检查，实际上这些内存缓冲区甚至不需要被注册，并且在ibv_post_send（）结束之后可以立即重用。仅对发送和RDMA写操作码有效。由于在该代码中没有涉及到key的交换，所以也无法使用RDMA传输，所以还是使用了CPU读取数据，既然是CPU读取，那么也就不需要注册内存缓冲区了，这个标记只能用于发送和写操作。
6、操作码和对应的QP传输服务类型的关系？