[Qemu-devel] [PULL 4/9] nbd: improve CMD_CACHE: use BDRV_REQ_PREFETCH

[Eric Blake <eblake@redhat.com>]

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

This helps to avoid extra io, allocations and memory copying.
We assume here that CMD_CACHE is always used with copy-on-read, as
otherwise it's a noop.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190725100550.33801-4-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 nbd/server.c | 43 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 35 insertions(+), 8 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 10faedcfc55d..a2cf085f7635 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -2104,12 +2104,15 @@ static int nbd_co_receive_request(NBDRequestData *req, NBDRequest *request,
             return -EINVAL;
         }

-        req->data = blk_try_blockalign(client->exp->blk, request->len);
-        if (req->data == NULL) {
-            error_setg(errp, "No memory");
-            return -ENOMEM;
+        if (request->type != NBD_CMD_CACHE) {
+            req->data = blk_try_blockalign(client->exp->blk, request->len);
+            if (req->data == NULL) {
+                error_setg(errp, "No memory");
+                return -ENOMEM;
+            }
         }
     }
+
     if (request->type == NBD_CMD_WRITE) {
         if (nbd_read(client->ioc, req->data, request->len, "CMD_WRITE data",
                      errp) < 0)
@@ -2194,7 +2197,7 @@ static coroutine_fn int nbd_do_cmd_read(NBDClient *client, NBDRequest *request,
     int ret;
     NBDExport *exp = client->exp;

-    assert(request->type == NBD_CMD_READ || request->type == NBD_CMD_CACHE);
+    assert(request->type == NBD_CMD_READ);

     /* XXX: NBD Protocol only documents use of FUA with WRITE */
     if (request->flags & NBD_CMD_FLAG_FUA) {
@@ -2206,7 +2209,7 @@ static coroutine_fn int nbd_do_cmd_read(NBDClient *client, NBDRequest *request,
     }

     if (client->structured_reply && !(request->flags & NBD_CMD_FLAG_DF) &&
-        request->len && request->type != NBD_CMD_CACHE)
+        request->len)
     {
         return nbd_co_send_sparse_read(client, request->handle, request->from,
                                        data, request->len, errp);
@@ -2214,7 +2217,7 @@ static coroutine_fn int nbd_do_cmd_read(NBDClient *client, NBDRequest *request,

     ret = blk_pread(exp->blk, request->from + exp->dev_offset, data,
                     request->len);
-    if (ret < 0 || request->type == NBD_CMD_CACHE) {
+    if (ret < 0) {
         return nbd_send_generic_reply(client, request->handle, ret,
                                       "reading from file failed", errp);
     }
@@ -2233,6 +2236,28 @@ static coroutine_fn int nbd_do_cmd_read(NBDClient *client, NBDRequest *request,
     }
 }

+/*
+ * nbd_do_cmd_cache
+ *
+ * Handle NBD_CMD_CACHE request.
+ * Return -errno if sending fails. Other errors are reported directly to the
+ * client as an error reply.
+ */
+static coroutine_fn int nbd_do_cmd_cache(NBDClient *client, NBDRequest *request,
+                                         Error **errp)
+{
+    int ret;
+    NBDExport *exp = client->exp;
+
+    assert(request->type == NBD_CMD_CACHE);
+
+    ret = blk_co_preadv(exp->blk, request->from + exp->dev_offset, request->len,
+                        NULL, BDRV_REQ_COPY_ON_READ | BDRV_REQ_PREFETCH);
+
+    return nbd_send_generic_reply(client, request->handle, ret,
+                                  "caching data failed", errp);
+}
+
 /* Handle NBD request.
  * Return -errno if sending fails. Other errors are reported directly to the
  * client as an error reply. */
@@ -2246,8 +2271,10 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
     char *msg;

     switch (request->type) {
-    case NBD_CMD_READ:
     case NBD_CMD_CACHE:
+        return nbd_do_cmd_cache(client, request, errp);
+
+    case NBD_CMD_READ:
         return nbd_do_cmd_read(client, request, data, errp);

     case NBD_CMD_WRITE:
-- 
2.20.1