nbd: Treat flags vs. command type as separate fields

Current upstream NBD documents that requests have a 16-bit flags,
followed by a 16-bit type integer; although older versions mentioned
only a 32-bit field with masking to find flags.  Since the protocol
is in network order (big-endian over the wire), the ABI is unchanged;
but dealing with the flags as a separate field rather than masking
will make it easier to add support for upcoming NBD extensions that
increase the number of both flags and commands.

Improve some comments in nbd.h based on the current upstream
NBD protocol (https://github.com/yoe/nbd/blob/master/doc/proto.md),
and touch some nearby code to keep checkpatch.pl happy.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1476469998-28592-3-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 19 insertions, 20 deletions

diff --git a/nbd/server.c b/nbd/server.c
index ac42391..38a0fef 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1,4 +1,5 @@
 /*
+ *  Copyright (C) 2016 Red Hat, Inc.
  *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
  *
  *  Network Block Device Server Side
@@ -652,21 +653,23 @@ static ssize_t nbd_receive_request(QIOChannel *ioc, struct nbd_request *request)
 
     /* Request
        [ 0 ..  3]   magic   (NBD_REQUEST_MAGIC)
-       [ 4 ..  7]   type    (0 == READ, 1 == WRITE)
+       [ 4 ..  5]   flags   (NBD_CMD_FLAG_FUA, ...)
+       [ 6 ..  7]   type    (NBD_CMD_READ, ...)
        [ 8 .. 15]   handle
        [16 .. 23]   from
        [24 .. 27]   len
      */
 
     magic = ldl_be_p(buf);
-    request->type   = ldl_be_p(buf + 4);
+    request->flags  = lduw_be_p(buf + 4);
+    request->type   = lduw_be_p(buf + 6);
     request->handle = ldq_be_p(buf + 8);
     request->from   = ldq_be_p(buf + 16);
     request->len    = ldl_be_p(buf + 24);
 
-    TRACE("Got request: { magic = 0x%" PRIx32 ", .type = %" PRIx32
-          ", from = %" PRIu64 " , len = %" PRIu32 " }",
-          magic, request->type, request->from, request->len);
+    TRACE("Got request: { magic = 0x%" PRIx32 ", .flags = %" PRIx16
+          ", .type = %" PRIx16 ", from = %" PRIu64 ", len = %" PRIu32 " }",
+          magic, request->flags, request->type, request->from, request->len);
 
     if (magic != NBD_REQUEST_MAGIC) {
         LOG("invalid magic (got 0x%" PRIx32 ")", magic);
@@ -1013,7 +1016,6 @@ static ssize_t nbd_co_receive_request(NBDRequest *req,
                                       struct nbd_request *request)
 {
     NBDClient *client = req->client;
-    uint32_t command;
     ssize_t rc;
 
     g_assert(qemu_in_coroutine());
@@ -1030,13 +1032,12 @@ static ssize_t nbd_co_receive_request(NBDRequest *req,
 
     TRACE("Decoding type");
 
-    command = request->type & NBD_CMD_MASK_COMMAND;
-    if (command != NBD_CMD_WRITE) {
+    if (request->type != NBD_CMD_WRITE) {
         /* No payload, we are ready to read the next request.  */
         req->complete = true;
     }
 
-    if (command == NBD_CMD_DISC) {
+    if (request->type == NBD_CMD_DISC) {
         /* Special case: we're going to disconnect without a reply,
          * whether or not flags, from, or len are bogus */
         TRACE("Request type is DISCONNECT");
@@ -1053,7 +1054,7 @@ static ssize_t nbd_co_receive_request(NBDRequest *req,
         goto out;
     }
 
-    if (command == NBD_CMD_READ || command == NBD_CMD_WRITE) {
+    if (request->type == NBD_CMD_READ || request->type == NBD_CMD_WRITE) {
         if (request->len > NBD_MAX_BUFFER_SIZE) {
             LOG("len (%" PRIu32" ) is larger than max len (%u)",
                 request->len, NBD_MAX_BUFFER_SIZE);
@@ -1067,7 +1068,7 @@ static ssize_t nbd_co_receive_request(NBDRequest *req,
             goto out;
         }
     }
-    if (command == NBD_CMD_WRITE) {
+    if (request->type == NBD_CMD_WRITE) {
         TRACE("Reading %" PRIu32 " byte(s)", request->len);
 
         if (read_sync(client->ioc, req->data, request->len) != request->len) {
@@ -1083,12 +1084,11 @@ static ssize_t nbd_co_receive_request(NBDRequest *req,
         LOG("operation past EOF; From: %" PRIu64 ", Len: %" PRIu32
             ", Size: %" PRIu64, request->from, request->len,
             (uint64_t)client->exp->size);
-        rc = command == NBD_CMD_WRITE ? -ENOSPC : -EINVAL;
+        rc = request->type == NBD_CMD_WRITE ? -ENOSPC : -EINVAL;
         goto out;
     }
-    if (request->type & ~NBD_CMD_MASK_COMMAND & ~NBD_CMD_FLAG_FUA) {
-        LOG("unsupported flags (got 0x%x)",
-            request->type & ~NBD_CMD_MASK_COMMAND);
+    if (request->flags & ~NBD_CMD_FLAG_FUA) {
+        LOG("unsupported flags (got 0x%x)", request->flags);
         rc = -EINVAL;
         goto out;
     }
@@ -1110,7 +1110,6 @@ static void nbd_trip(void *opaque)
     struct nbd_request request;
     struct nbd_reply reply;
     ssize_t ret;
-    uint32_t command;
     int flags;
 
     TRACE("Reading request.");
@@ -1134,7 +1133,6 @@ static void nbd_trip(void *opaque)
         reply.error = -ret;
         goto error_reply;
     }
-    command = request.type & NBD_CMD_MASK_COMMAND;
 
     if (client->closing) {
         /*
@@ -1144,11 +1142,12 @@ static void nbd_trip(void *opaque)
         goto done;
     }
 
-    switch (command) {
+    switch (request.type) {
     case NBD_CMD_READ:
         TRACE("Request type is READ");
 
-        if (request.type & NBD_CMD_FLAG_FUA) {
+        /* XXX: NBD Protocol only documents use of FUA with WRITE */
+        if (request.flags & NBD_CMD_FLAG_FUA) {
             ret = blk_co_flush(exp->blk);
             if (ret < 0) {
                 LOG("flush failed");
@@ -1181,7 +1180,7 @@ static void nbd_trip(void *opaque)
         TRACE("Writing to device");
 
         flags = 0;
-        if (request.type & NBD_CMD_FLAG_FUA) {
+        if (request.flags & NBD_CMD_FLAG_FUA) {
             flags |= BDRV_REQ_FUA;
         }
         ret = blk_pwrite(exp->blk, request.from + exp->dev_offset,