[PATCH 2/2] migration/rdma: advise prefetch write for ODP region

[Li Zhijian]

The responder mr registering with ODP will sent RNR NAK back to
the requester in the face of the page fault.
---------
ibv_poll_cq wc.status=13 RNR retry counter exceeded!
ibv_poll_cq wrid=WRITE RDMA!
---------
ibv_advise_mr(3) helps to make pages present before the actual IO is
conducted so that the responder does page fault as little as possible.

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
---
 migration/rdma.c       | 40 ++++++++++++++++++++++++++++++++++++++++
 migration/trace-events |  1 +
 2 files changed, 41 insertions(+)

diff --git a/migration/rdma.c b/migration/rdma.c
index 8784b5f22a6..a2ad00d665f 100644
--- a/migration/rdma.c
+++ b/migration/rdma.c
@@ -1117,6 +1117,30 @@ static int qemu_rdma_alloc_qp(RDMAContext *rdma)
     return 0;
 }
 
+/*
+ * ibv_advise_mr to avoid RNR NAK error as far as possible.
+ * The responder mr registering with ODP will sent RNR NAK back to
+ * the requester in the face of the page fault.
+ */
+static void qemu_rdma_advise_prefetch_write_mr(struct ibv_pd *pd, uint64_t 
addr,
+                                               uint32_t len,  uint32_t lkey,
+                                               const char *name, bool wr)
+{
+    int ret;
+    int advice = wr ? IBV_ADVISE_MR_ADVICE_PREFETCH_WRITE :
+                 IBV_ADVISE_MR_ADVICE_PREFETCH;
+    struct ibv_sge sg_list = {.lkey = lkey, .addr = addr, .length = len};
+
+    ret = ibv_advise_mr(pd, advice,
+                        IB_UVERBS_ADVISE_MR_FLAG_FLUSH, &sg_list, 1);
+    /* ignore the error */
+    if (ret) {
+        trace_qemu_rdma_advise_mr(name, len, addr, strerror(errno));
+    } else {
+        trace_qemu_rdma_advise_mr(name, len, addr, "successed");
+    }
+}
+
 static int qemu_rdma_reg_whole_ram_blocks(RDMAContext *rdma)
 {
     int i;
@@ -1140,6 +1164,17 @@ on_demand:
             perror("Failed to register local dest ram block!\n");
             break;
         }
+
+        if (access & IBV_ACCESS_ON_DEMAND) {
+            qemu_rdma_advise_prefetch_write_mr(rdma->pd,
+                                               (uintptr_t)
+                                               local->block[i].local_host_addr,
+                                               local->block[i].length,
+                                               local->block[i].mr->lkey,
+                                               local->block[i].block_name,
+                                               true);
+        }
+
         rdma->total_registrations++;
     }
 
@@ -1244,6 +1279,11 @@ on_demand:
                             rdma->total_registrations);
             return -1;
         }
+        if (access & IBV_ACCESS_ON_DEMAND) {
+            qemu_rdma_advise_prefetch_write_mr(rdma->pd, 
(uintptr_t)chunk_start,
+                                               len, block->pmr[chunk]->lkey,
+                                               block->block_name, rkey);
+        }
         rdma->total_registrations++;
     }
 
diff --git a/migration/trace-events b/migration/trace-events
index 5f6aa580def..901c1d54c12 100644
--- a/migration/trace-events
+++ b/migration/trace-events
@@ -213,6 +213,7 @@ qemu_rdma_poll_other(const char *compstr, int64_t comp, int 
left) "other complet
 qemu_rdma_post_send_control(const char *desc) "CONTROL: sending %s.."
 qemu_rdma_register_and_get_keys(uint64_t len, void *start) "Registering %" 
PRIu64 " bytes @ %p"
 qemu_rdma_register_odp_mr(const char *name) "Try to register On-Demand Paging 
memory region: %s"
+qemu_rdma_advise_mr(const char *name, uint32_t len, uint64_t addr, const char 
*res) "Try to advise block %s prefetch write at %" PRIu32 "@0x%" PRIx64 ": %s"
 qemu_rdma_registration_handle_compress(int64_t length, int index, int64_t 
offset) "Zapping zero chunk: %" PRId64 " bytes, index %d, offset %" PRId64
 qemu_rdma_registration_handle_finished(void) ""
 qemu_rdma_registration_handle_ram_blocks(void) ""
-- 
2.31.1