[PATCH v2 1/4] nbd: Add multi-conn option

[Eric Blake]

From: "Richard W.M. Jones" <rjo...@redhat.com>

Add multi-conn option to the NBD client.  This commit just adds the
option, it is not functional.

Setting this to a value > 1 permits multiple connections to the NBD
server; a typical value might be 4.  The default is 1, meaning only a
single connection is made.  If the NBD server does not advertise that
it is safe for multi-conn then this setting is forced to 1.

Signed-off-by: Richard W.M. Jones <rjo...@redhat.com>
[eblake: also expose it through QMP]
Signed-off-by: Eric Blake <ebl...@redhat.com>
---
 qapi/block-core.json |  8 +++++++-
 block/nbd.c          | 24 ++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/qapi/block-core.json b/qapi/block-core.json
index 7f70ec6d3cb..5c10824f35b 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -4545,6 +4545,11 @@
 #     until successful or until @open-timeout seconds have elapsed.
 #     Default 0 (Since 7.0)
 #
+# @multi-conn: Request the number of parallel client connections to make
+#     to the server, up to 16.  If the server does not advertise support
+#     for multiple connections, or if this value is 0 or 1, all traffic
+#     is sent through a single connection.  Default 1 (Since 10.1)
+#
 # Features:
 #
 # @unstable: Member @x-dirty-bitmap is experimental.
@@ -4558,7 +4563,8 @@
             '*tls-hostname': 'str',
             '*x-dirty-bitmap': { 'type': 'str', 'features': [ 'unstable' ] },
             '*reconnect-delay': 'uint32',
-            '*open-timeout': 'uint32' } }
+            '*open-timeout': 'uint32',
+            '*multi-conn': 'uint32' } }

 ##
 # @BlockdevOptionsRaw:
diff --git a/block/nbd.c b/block/nbd.c
index d5a2b21c6d1..5eb00e360af 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -48,6 +48,7 @@

 #define EN_OPTSTR ":exportname="
 #define MAX_NBD_REQUESTS    16
+#define MAX_MULTI_CONN      16

 #define COOKIE_TO_INDEX(cookie) ((cookie) - 1)
 #define INDEX_TO_COOKIE(index)  ((index) + 1)
@@ -97,6 +98,7 @@ typedef struct BDRVNBDState {
     /* Connection parameters */
     uint32_t reconnect_delay;
     uint32_t open_timeout;
+    uint32_t multi_conn;
     SocketAddress *saddr;
     char *export;
     char *tlscredsid;
@@ -1840,6 +1842,15 @@ static QemuOptsList nbd_runtime_opts = {
                     "attempts until successful or until @open-timeout seconds "
                     "have elapsed. Default 0",
         },
+        {
+            .name = "multi-conn",
+            .type = QEMU_OPT_NUMBER,
+            .help = "If > 1 permit up to this number of connections to the "
+                    "server. The server must also advertise multi-conn "
+                    "support.  If <= 1, only a single connection is made "
+                    "to the server even if the server advertises multi-conn. "
+                    "Default 1",
+        },
         { /* end of list */ }
     },
 };
@@ -1895,6 +1906,10 @@ static int nbd_process_options(BlockDriverState *bs, 
QDict *options,

     s->reconnect_delay = qemu_opt_get_number(opts, "reconnect-delay", 0);
     s->open_timeout = qemu_opt_get_number(opts, "open-timeout", 0);
+    s->multi_conn = qemu_opt_get_number(opts, "multi-conn", 1);
+    if (s->multi_conn > MAX_MULTI_CONN) {
+        s->multi_conn = MAX_MULTI_CONN;
+    }

     ret = 0;

@@ -1949,6 +1964,15 @@ static int nbd_open(BlockDriverState *bs, QDict 
*options, int flags,

     nbd_client_connection_enable_retry(s->conn);

+    /*
+     * We set s->multi_conn in nbd_process_options above, but now that
+     * we have connected if the server doesn't advertise that it is
+     * safe for multi-conn, force it to 1.
+     */
+    if (!(s->info.flags & NBD_FLAG_CAN_MULTI_CONN)) {
+        s->multi_conn = 1;
+    }
+
     return 0;

 fail:
-- 
2.49.0