nbd/client: Add x-dirty-bitmap to query bitmap from server

In order to test that the NBD server is properly advertising
dirty bitmaps, we need a bare minimum client that can request
and read the context.  Since feature freeze for 3.0 is imminent,
this is the smallest workable patch, which replaces the qemu
block status report with the results of the NBD server's dirty
bitmap (making it very easy to use 'qemu-img map --output=json'
to learn where the dirty portions are).  Note that the NBD
protocol defines a dirty section with the same bit but opposite
sense that normal "base:allocation" uses to report an allocated
section; so in qemu-img map output, "data":true corresponds to
clean, "data":false corresponds to dirty.

A more complete solution that allows dirty bitmaps to be queried
at the same time as normal block status will be required before
this addition can lose the x- prefix.  Until then, the fact that
this replaces normal status with dirty status means actions
like 'qemu-img convert' will likely misbehave due to treating
dirty regions of the file as if they are unallocated.

The next patch adds an iotest to exercise this new code.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180702191458.28741-2-eblake@redhat.com>

6 files changed, 21 insertions, 5 deletions

diff --git a/block/nbd-client.c b/block/nbd-client.c
index 8d69eaa..9686ecb 100644
--- a/block/nbd-client.c
+++ b/block/nbd-client.c
@@ -970,6 +970,7 @@ int nbd_client_init(BlockDriverState *bs,
                     const char *export,
                     QCryptoTLSCreds *tlscreds,
                     const char *hostname,
+                    const char *x_dirty_bitmap,
                     Error **errp)
 {
     NBDClientSession *client = nbd_get_client_session(bs);
@@ -982,9 +983,11 @@ int nbd_client_init(BlockDriverState *bs,
     client->info.request_sizes = true;
     client->info.structured_reply = true;
     client->info.base_allocation = true;
+    client->info.x_dirty_bitmap = g_strdup(x_dirty_bitmap);
     ret = nbd_receive_negotiate(QIO_CHANNEL(sioc), export,
                                 tlscreds, hostname,
                                 &client->ioc, &client->info, errp);
+    g_free(client->info.x_dirty_bitmap);
     if (ret < 0) {
         logout("Failed to negotiate with the NBD server\n");
         return ret;
diff --git a/block/nbd-client.h b/block/nbd-client.h
index 0ece76e..cfc9055 100644
--- a/block/nbd-client.h
+++ b/block/nbd-client.h
@@ -45,6 +45,7 @@ int nbd_client_init(BlockDriverState *bs,
                     const char *export_name,
                     QCryptoTLSCreds *tlscreds,
                     const char *hostname,
+                    const char *x_dirty_bitmap,
                     Error **errp);
 void nbd_client_close(BlockDriverState *bs);
 
diff --git a/block/nbd.c b/block/nbd.c
index 13db403..b198ad7 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -378,6 +378,12 @@ static QemuOptsList nbd_runtime_opts = {
             .type = QEMU_OPT_STRING,
             .help = "ID of the TLS credentials to use",
         },
+        {
+            .name = "x-dirty-bitmap",
+            .type = QEMU_OPT_STRING,
+            .help = "experimental: expose named dirty bitmap in place of "
+                    "block status",
+        },
         { /* end of list */ }
     },
 };
@@ -438,8 +444,8 @@ static int nbd_open(BlockDriverState *bs, QDict *options, int flags,
     }
 
     /* NBD handshake */
-    ret = nbd_client_init(bs, sioc, s->export,
-                          tlscreds, hostname, errp);
+    ret = nbd_client_init(bs, sioc, s->export, tlscreds, hostname,
+                          qemu_opt_get(opts, "x-dirty-bitmap"), errp);
  error:
     if (sioc) {
         object_unref(OBJECT(sioc));
diff --git a/include/block/nbd.h b/include/block/nbd.h
index daaeae6..4638c83 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -259,6 +259,7 @@ static inline bool nbd_reply_type_is_error(int type)
 struct NBDExportInfo {
     /* Set by client before nbd_receive_negotiate() */
     bool request_sizes;
+    char *x_dirty_bitmap;
 
     /* In-out fields, set by client before nbd_receive_negotiate() and
      * updated by server results during nbd_receive_negotiate() */
diff --git a/nbd/client.c b/nbd/client.c
index 232ff4f..40b74d9 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -1,5 +1,5 @@
 /*
- *  Copyright (C) 2016-2017 Red Hat, Inc.
+ *  Copyright (C) 2016-2018 Red Hat, Inc.
  *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
  *
  *  Network Block Device Client Side
@@ -831,7 +831,7 @@ int nbd_receive_negotiate(QIOChannel *ioc, const char *name,
 
             if (info->structured_reply && base_allocation) {
                 result = nbd_negotiate_simple_meta_context(
-                        ioc, name, "base:allocation",
+                        ioc, name, info->x_dirty_bitmap ?: "base:allocation",
                         &info->meta_base_allocation_id, errp);
                 if (result < 0) {
                     goto fail;
diff --git a/qapi/block-core.json b/qapi/block-core.json
index 577ce5e..90e554e 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -3471,12 +3471,17 @@
 #
 # @tls-creds:   TLS credentials ID
 #
+# @x-dirty-bitmap: A "qemu:dirty-bitmap:NAME" string to query in place of
+#                  traditional "base:allocation" block status (see
+#                  NBD_OPT_LIST_META_CONTEXT in the NBD protocol) (since 3.0)
+#
 # Since: 2.9
 ##
 { 'struct': 'BlockdevOptionsNbd',
   'data': { 'server': 'SocketAddress',
             '*export': 'str',
-            '*tls-creds': 'str' } }
+            '*tls-creds': 'str',
+            '*x-dirty-bitmap': 'str' } }
 
 ##
 # @BlockdevOptionsRaw: