nbd: Add new qemu:allocation-depth metadata context

'qemu-img map' provides a way to determine which extents of an image
come from the top layer vs. inherited from a backing chain.  This is
useful information worth exposing over NBD.  There is a proposal to
add a QMP command block-dirty-bitmap-populate which can create a dirty
bitmap that reflects allocation information, at which point the
qemu:dirty-bitmap:NAME metadata context can expose that information
via the creation of a temporary bitmap, but we can shorten the effort
by adding a new qemu:allocation-depth metadata context that does the
same thing without an intermediate bitmap (this patch does not
eliminate the need for that proposal, as it will have other uses as
well).

While documenting things, remember that although the NBD protocol has
NBD_OPT_SET_META_CONTEXT, the rest of its documentation refers to
'metadata context', which is a more apt description of what is
actually being used by NBD_CMD_BLOCK_STATUS: the user is requesting
metadata by passing one or more context names.  So I also touched up
some existing wording to prefer the term 'metadata context' where it
makes sense.

Note that this patch does not actually enable any way to request a
server to enable this context; that will come in the next patch.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20201027050556.269064-10-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/include/block/nbd.h b/include/block/nbd.h
index 3dd9a04546..4a52a43ef5 100644
--- a/include/block/nbd.h
+++ b/include/block/nbd.h
@@ -1,5 +1,5 @@
 /*
- *  Copyright (C) 2016-2019 Red Hat, Inc.
+ *  Copyright (C) 2016-2020 Red Hat, Inc.
  *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
  *
  *  Network Block Device
@@ -47,7 +47,7 @@ typedef struct NBDOptionReply NBDOptionReply;
 typedef struct NBDOptionReplyMetaContext {
     NBDOptionReply h; /* h.type = NBD_REP_META_CONTEXT, h.length > 4 */
     uint32_t context_id;
-    /* meta context name follows */
+    /* metadata context name follows */
 } QEMU_PACKED NBDOptionReplyMetaContext;
 
 /* Transmission phase structs
@@ -229,7 +229,7 @@ enum {
 #define NBD_MAX_BUFFER_SIZE (32 * 1024 * 1024)
 
 /*
- * Maximum size of a protocol string (export name, meta context name,
+ * Maximum size of a protocol string (export name, metadata context name,
  * etc.).  Use malloc rather than stack allocation for storage of a
  * string.
  */
@@ -259,6 +259,8 @@ enum {
 /* Extent flags for qemu:dirty-bitmap in NBD_REPLY_TYPE_BLOCK_STATUS */
 #define NBD_STATE_DIRTY (1 << 0)
 
+/* No flags needed for qemu:allocation-depth in NBD_REPLY_TYPE_BLOCK_STATUS */
+
 static inline bool nbd_reply_type_is_error(int type)
 {
     return type & (1 << 15);