nbd: Support shorter handshake

The NBD Protocol allows the server and client to mutually agree
on a shorter handshake (omit the 124 bytes of reserved 0), via
the server advertising NBD_FLAG_NO_ZEROES and the client
acknowledging with NBD_FLAG_C_NO_ZEROES (only possible in
newstyle, whether or not it is fixed newstyle).  It doesn't
shave much off the wire, but we might as well implement it.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NAlex Bligh <alex@alex.org.uk>
Message-Id: <1476469998-28592-13-git-send-email-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

include/block/nbd.h

@@ -74,11 +74,13 @@ typedef struct NBDReply NBDReply;
 
 /* New-style handshake (global) flags, sent from server to client, and
    control what will happen during handshake phase. */
-#define NBD_FLAG_FIXED_NEWSTYLE     (1 << 0)    /* Fixed newstyle protocol. */
+#define NBD_FLAG_FIXED_NEWSTYLE   (1 << 0) /* Fixed newstyle protocol. */
+#define NBD_FLAG_NO_ZEROES        (1 << 1) /* End handshake without zeroes. */
 
 /* New-style client flags, sent from client to server to control what happens
    during handshake phase. */
-#define NBD_FLAG_C_FIXED_NEWSTYLE   (1 << 0)    /* Fixed newstyle protocol. */
+#define NBD_FLAG_C_FIXED_NEWSTYLE (1 << 0) /* Fixed newstyle protocol. */
+#define NBD_FLAG_C_NO_ZEROES      (1 << 1) /* End handshake without zeroes. */
 
 /* Reply types. */
 #define NBD_REP_ACK             (1)             /* Data sending finished. */

nbd/client.c

@@ -440,6 +440,7 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint16_t *flags,
     char buf[256];
     uint64_t magic, s;
     int rc;
+    bool zeroes = true;
 
     TRACE("Receiving negotiation tlscreds=%p hostname=%s.",
           tlscreds, hostname ? hostname : "<null>");
@@ -504,6 +505,11 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint16_t *flags,
             TRACE("Server supports fixed new style");
             clientflags |= NBD_FLAG_C_FIXED_NEWSTYLE;
         }
+        if (globalflags & NBD_FLAG_NO_ZEROES) {
+            zeroes = false;
+            TRACE("Server supports no zeroes");
+            clientflags |= NBD_FLAG_C_NO_ZEROES;
+        }
         /* client requested flags */
         clientflags = cpu_to_be32(clientflags);
         if (write_sync(ioc, &clientflags, sizeof(clientflags)) !=
@@ -591,7 +597,7 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name, uint16_t *flags,
     }
 
     TRACE("Size is %" PRIu64 ", export flags %" PRIx16, *size, *flags);
-    if (drop_sync(ioc, 124) != 124) {
+    if (zeroes && drop_sync(ioc, 124) != 124) {
         error_setg(errp, "Failed to read reserved block");
         goto fail;
     }

nbd/server.c

@@ -81,6 +81,7 @@ struct NBDClient {
     int refcount;
     void (*close)(NBDClient *client);
 
+    bool no_zeroes;
     NBDExport *exp;
     QCryptoTLSCreds *tlscreds;
     char *tlsaclname;
@@ -450,6 +451,11 @@ static int nbd_negotiate_options(NBDClient *client)
         fixedNewstyle = true;
         flags &= ~NBD_FLAG_C_FIXED_NEWSTYLE;
     }
+    if (flags & NBD_FLAG_C_NO_ZEROES) {
+        TRACE("Client supports no zeroes at handshake end");
+        client->no_zeroes = true;
+        flags &= ~NBD_FLAG_C_NO_ZEROES;
+    }
     if (flags != 0) {
         TRACE("Unknown client flags 0x%" PRIx32 " received", flags);
         return -EIO;
@@ -602,6 +608,7 @@ static coroutine_fn int nbd_negotiate(NBDClientNewData *data)
     const uint16_t myflags = (NBD_FLAG_HAS_FLAGS | NBD_FLAG_SEND_TRIM |
                               NBD_FLAG_SEND_FLUSH | NBD_FLAG_SEND_FUA);
     bool oldStyle;
+    size_t len;
 
     /* Old style negotiation header without options
         [ 0 ..   7]   passwd       ("NBDMAGIC")
@@ -618,7 +625,7 @@ static coroutine_fn int nbd_negotiate(NBDClientNewData *data)
         ....options sent....
         [18 ..  25]   size
         [26 ..  27]   export flags
-        [28 .. 151]   reserved     (0)
+        [28 .. 151]   reserved     (0, omit if no_zeroes)
      */
 
     qio_channel_set_blocking(client->ioc, false, NULL);
@@ -637,7 +644,7 @@ static coroutine_fn int nbd_negotiate(NBDClientNewData *data)
         stw_be_p(buf + 26, client->exp->nbdflags | myflags);
     } else {
         stq_be_p(buf + 8, NBD_OPTS_MAGIC);
-        stw_be_p(buf + 16, NBD_FLAG_FIXED_NEWSTYLE);
+        stw_be_p(buf + 16, NBD_FLAG_FIXED_NEWSTYLE | NBD_FLAG_NO_ZEROES);
     }
 
     if (oldStyle) {
@@ -664,8 +671,8 @@ static coroutine_fn int nbd_negotiate(NBDClientNewData *data)
               client->exp->size, client->exp->nbdflags | myflags);
         stq_be_p(buf + 18, client->exp->size);
         stw_be_p(buf + 26, client->exp->nbdflags | myflags);
-        if (nbd_negotiate_write(client->ioc, buf + 18, sizeof(buf) - 18) !=
-            sizeof(buf) - 18) {
+        len = client->no_zeroes ? 10 : sizeof(buf) - 18;
+        if (nbd_negotiate_write(client->ioc, buf + 18, len) != len) {
             LOG("write failed");
             goto fail;
         }