Re: [Qemu-devel] [PATCH v2 3/7] block/nbd-client: refactor reading reply

[Vladimir Sementsov-Ogievskiy]

18.09.2017 18:43, Paolo Bonzini wrote:
On 18/09/2017 15:59, Vladimir Sementsov-Ogievskiy wrote:
Read the whole reply in one place - in nbd_read_reply_entry.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
---
  block/nbd-client.h |  1 +
  block/nbd-client.c | 42 ++++++++++++++++++++++++------------------
  2 files changed, 25 insertions(+), 18 deletions(-)

diff --git a/block/nbd-client.h b/block/nbd-client.h
index b1900e6a6d..3f97d31013 100644
--- a/block/nbd-client.h
+++ b/block/nbd-client.h
@@ -21,6 +21,7 @@ typedef struct {
      Coroutine *coroutine;
      bool receiving;         /* waiting for read_reply_co? */
      NBDRequest *request;
+    QEMUIOVector *qiov;
  } NBDClientRequest;
  
  typedef struct NBDClientSession {
diff --git a/block/nbd-client.c b/block/nbd-client.c
index 5f241ecc22..f7b642f079 100644
--- a/block/nbd-client.c
+++ b/block/nbd-client.c
@@ -98,6 +98,21 @@ static coroutine_fn void nbd_read_reply_entry(void *opaque)
              break;
          }
  
+        if (s->reply.error == 0 &&
+            s->requests[i].request->type == NBD_CMD_READ)
+        {
+            QEMUIOVector *qiov = s->requests[i].qiov;
+            assert(qiov != NULL);
+            assert(s->requests[i].request->len ==
+                   iov_size(qiov->iov, qiov->niov));
+            if (qio_channel_readv_all(s->ioc, qiov->iov, qiov->niov,
+                                      NULL) < 0)
+            {
+                s->reply.error = EIO;
+                break;
+            }
+        }
I am not sure this is an improvement.  In principle you could have
commands that read replies a bit at a time without using a QEMUIOVector.

Paolo

Hm, what do you mean? In this patch I don't change "how do we read it", 
I only move the reading code to one coroutine, to make it simple to 
extend it in future (block status, etc). nbd_read_reply_enty has all 
information it need (s->requests[i].request) to handle the whole reply.. 
It's an improvement, because it leads to separating reply_entry 
coroutine - it don't need any synchronization (yield/wake) more with 
request coroutines.


          /* We're woken up again by the request itself.  Note that there
           * is no race between yielding and reentering read_reply_co.  This
           * is because:
@@ -118,6 +133,7 @@ static coroutine_fn void nbd_read_reply_entry(void *opaque)
      s->read_reply_co = NULL;
  }
  
+/* qiov should be provided for READ and WRITE */
  static int nbd_co_send_request(BlockDriverState *bs,
                                 NBDRequest *request,
                                 QEMUIOVector *qiov)
@@ -145,6 +161,7 @@ static int nbd_co_send_request(BlockDriverState *bs,
  
      request->handle = INDEX_TO_HANDLE(s, i);
      s->requests[i].request = request;
+    s->requests[i].qiov = qiov;
  
      if (s->quit) {
          rc = -EIO;
@@ -155,7 +172,8 @@ static int nbd_co_send_request(BlockDriverState *bs,
          goto err;
      }
  
-    if (qiov) {
+    if (s->requests[i].request->type == NBD_CMD_WRITE) {
+        assert(qiov);
          qio_channel_set_cork(s->ioc, true);
          rc = nbd_send_request(s->ioc, request);
          if (rc >= 0 && !s->quit) {
@@ -181,9 +199,7 @@ err:
      return rc;
  }
  
-static int nbd_co_receive_reply(NBDClientSession *s,
-                                NBDRequest *request,
-                                QEMUIOVector *qiov)
+static int nbd_co_receive_reply(NBDClientSession *s, NBDRequest *request)
  {
      int ret;
      int i = HANDLE_TO_INDEX(s, request->handle);
@@ -197,14 +213,6 @@ static int nbd_co_receive_reply(NBDClientSession *s,
      } else {
          assert(s->reply.handle == request->handle);
          ret = -s->reply.error;
-        if (qiov && s->reply.error == 0) {
-            assert(request->len == iov_size(qiov->iov, qiov->niov));
-            if (qio_channel_readv_all(s->ioc, qiov->iov, qiov->niov,
-                                      NULL) < 0) {
-                ret = -EIO;
-                s->quit = true;
-            }
-        }
  
          /* Tell the read handler to read another header.  */
          s->reply.handle = 0;
@@ -232,16 +240,14 @@ static int nbd_co_request(BlockDriverState *bs,
      NBDClientSession *client = nbd_get_client_session(bs);
      int ret;
  
-    assert(!qiov || request->type == NBD_CMD_WRITE ||
-           request->type == NBD_CMD_READ);
-    ret = nbd_co_send_request(bs, request,
-                              request->type == NBD_CMD_WRITE ? qiov : NULL);
+    assert((qiov == NULL) !=
+           (request->type == NBD_CMD_WRITE || request->type == NBD_CMD_READ));
+    ret = nbd_co_send_request(bs, request, qiov);
      if (ret < 0) {
          return ret;
      }
  
-    return nbd_co_receive_reply(client, request,
-                                request->type == NBD_CMD_READ ? qiov : NULL);
+    return nbd_co_receive_reply(client, request);
  }
  
  int nbd_client_co_preadv(BlockDriverState *bs, uint64_t offset,


--
Best regards,
Vladimir