[PATCH v3 1/6] nbd: Add new qemu:allocation-depth metacontext

[Eric Blake]

'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
qemu:dirty-bitmap:NAME can expose that information via the creation of
a temporary bitmap, but we can shorten the effort by adding a new
qemu:allocation-depth 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).

For this patch, I just encoded a tri-state value (unallocated, from
this layer, from any of the backing layers); an obvious extension
would be to provide the actual depth in bits 31-4 while keeping bits
1-0 as a tri-state (leaving bits 3-2 unused, for ease of reading depth
from a hex number).  But this extension would require
bdrv_is_allocated_above to return a depth number.

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>
---
 docs/interop/nbd.txt | 23 +++++++++--
 include/block/nbd.h  |  8 +++-
 nbd/server.c         | 92 ++++++++++++++++++++++++++++++++++++++++----
 3 files changed, 111 insertions(+), 12 deletions(-)

diff --git a/docs/interop/nbd.txt b/docs/interop/nbd.txt
index f3b3cacc9621..ea1ee94cac9b 100644
--- a/docs/interop/nbd.txt
+++ b/docs/interop/nbd.txt
@@ -17,9 +17,9 @@ namespace "qemu".

 == "qemu" namespace ==

-The "qemu" namespace currently contains only one type of context,
-related to exposing the contents of a dirty bitmap alongside the
-associated disk contents.  That context has the following form:
+The "qemu" namespace currently contains two types of context.  The
+first is related to exposing the contents of a dirty bitmap alongside
+the associated disk contents.  That context has the following form:

     qemu:dirty-bitmap:<dirty-bitmap-export-name>

@@ -28,8 +28,22 @@ in reply for NBD_CMD_BLOCK_STATUS:

     bit 0: NBD_STATE_DIRTY, means that the extent is "dirty"

+The second is related to exposing the source of various extents within
+the image, with a single context named:
+
+    qemu:allocation-depth
+
+In the allocation depth context, bits 0 and 1 form a tri-state value:
+
+    bits 0-1 clear: NBD_STATE_DEPTH_UNALLOC, the extent is unallocated
+    bit 0 set: NBD_STATE_DEPTH_LOCAL, the extent is allocated in the
+               top level of the image
+    bit 1 set: NBD_STATE_DEPTH_BACKING, the extent is inherited from a
+               backing layer
+
 For NBD_OPT_LIST_META_CONTEXT the following queries are supported
-in addition to "qemu:dirty-bitmap:<dirty-bitmap-export-name>":
+in addition to the specific "qemu:allocation-depth" and
+"qemu:dirty-bitmap:<dirty-bitmap-export-name>":

 * "qemu:" - returns list of all available metadata contexts in the
             namespace.
@@ -55,3 +69,4 @@ the operation of that feature.
 NBD_CMD_BLOCK_STATUS for "qemu:dirty-bitmap:", NBD_CMD_CACHE
 * 4.2: NBD_FLAG_CAN_MULTI_CONN for shareable read-only exports,
 NBD_CMD_FLAG_FAST_ZERO
+* 5.2: NBD_CMD_BLOCK_STATUS for "qemu:allocation-depth"
diff --git a/include/block/nbd.h b/include/block/nbd.h
index 3dd9a04546ec..06208bc25027 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
@@ -259,6 +259,12 @@ enum {
 /* Extent flags for qemu:dirty-bitmap in NBD_REPLY_TYPE_BLOCK_STATUS */
 #define NBD_STATE_DIRTY (1 << 0)

+/* Extent flags for qemu:allocation-depth in NBD_REPLY_TYPE_BLOCK_STATUS */
+#define NBD_STATE_DEPTH_UNALLOC (0 << 0)
+#define NBD_STATE_DEPTH_LOCAL   (1 << 0)
+#define NBD_STATE_DEPTH_BACKING (2 << 0)
+#define NBD_STATE_DEPTH_MASK    (0x3)
+
 static inline bool nbd_reply_type_is_error(int type)
 {
     return type & (1 << 15);
diff --git a/nbd/server.c b/nbd/server.c
index e75c825879aa..59533090f5ce 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -27,7 +27,8 @@
 #include "qemu/units.h"

 #define NBD_META_ID_BASE_ALLOCATION 0
-#define NBD_META_ID_DIRTY_BITMAP 1
+#define NBD_META_ID_ALLOCATION_DEPTH 1
+#define NBD_META_ID_DIRTY_BITMAP 2

 /*
  * NBD_MAX_BLOCK_STATUS_EXTENTS: 1 MiB of extents data. An empirical
@@ -94,6 +95,7 @@ struct NBDExport {
     BlockBackend *eject_notifier_blk;
     Notifier eject_notifier;

+    bool alloc_context;
     BdrvDirtyBitmap *export_bitmap;
     char *export_bitmap_context;
 };
@@ -108,6 +110,7 @@ typedef struct NBDExportMetaContexts {
     bool valid; /* means that negotiation of the option finished without
                    errors */
     bool base_allocation; /* export base:allocation context (block status) */
+    bool allocation_depth; /* export qemu:allocation-depth */
     bool bitmap; /* export qemu:dirty-bitmap:<export bitmap name> */
 } NBDExportMetaContexts;

@@ -852,7 +855,8 @@ static bool nbd_meta_base_query(NBDClient *client, 
NBDExportMetaContexts *meta,
 /* nbd_meta_qemu_query
  *
  * Handle queries to 'qemu' namespace. For now, only the qemu:dirty-bitmap:
- * context is available.  Return true if @query has been handled.
+ * and qemu:allocation-depth contexts are available.  Return true if @query
+ * has been handled.
  */
 static bool nbd_meta_qemu_query(NBDClient *client, NBDExportMetaContexts *meta,
                                 const char *query)
@@ -864,12 +868,19 @@ static bool nbd_meta_qemu_query(NBDClient *client, 
NBDExportMetaContexts *meta,

     if (!*query) {
         if (client->opt == NBD_OPT_LIST_META_CONTEXT) {
+            meta->allocation_depth = meta->exp->alloc_context;
             meta->bitmap = !!meta->exp->export_bitmap;
         }
         trace_nbd_negotiate_meta_query_parse("empty");
         return true;
     }

+    if (strcmp(query, "allocation-depth") == 0) {
+        trace_nbd_negotiate_meta_query_parse("allocation-depth");
+        meta->allocation_depth = meta->exp->alloc_context;
+        return true;
+    }
+
     if (nbd_strshift(&query, "dirty-bitmap:")) {
         trace_nbd_negotiate_meta_query_parse("dirty-bitmap:");
         if (!meta->exp->export_bitmap) {
@@ -884,7 +895,7 @@ static bool nbd_meta_qemu_query(NBDClient *client, 
NBDExportMetaContexts *meta,
         return true;
     }

-    trace_nbd_negotiate_meta_query_skip("not dirty-bitmap");
+    trace_nbd_negotiate_meta_query_skip("unknown qemu context");
     return true;
 }

@@ -984,6 +995,7 @@ static int nbd_negotiate_meta_queries(NBDClient *client,
     if (client->opt == NBD_OPT_LIST_META_CONTEXT && !nb_queries) {
         /* enable all known contexts */
         meta->base_allocation = true;
+        meta->allocation_depth = meta->exp->alloc_context;
         meta->bitmap = !!meta->exp->export_bitmap;
     } else {
         for (i = 0; i < nb_queries; ++i) {
@@ -1003,6 +1015,15 @@ static int nbd_negotiate_meta_queries(NBDClient *client,
         }
     }

+    if (meta->allocation_depth) {
+        ret = nbd_negotiate_send_meta_context(client, "qemu:allocation-depth",
+                                              NBD_META_ID_ALLOCATION_DEPTH,
+                                              errp);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
     if (meta->bitmap) {
         ret = nbd_negotiate_send_meta_context(client,
                                               meta->exp->export_bitmap_context,
@@ -1961,6 +1982,40 @@ static int blockstatus_to_extents(BlockDriverState *bs, 
uint64_t offset,
     return 0;
 }

+static int blockalloc_to_extents(BlockDriverState *bs, uint64_t offset,
+                                 uint64_t bytes, NBDExtentArray *ea)
+{
+    while (bytes) {
+        uint32_t flags;
+        int64_t num;
+        int ret = bdrv_is_allocated(bs, offset, bytes, &num);
+
+        if (ret < 0) {
+            return ret;
+        }
+
+        if (ret == 1) {
+            flags = NBD_STATE_DEPTH_LOCAL;
+        } else {
+            ret = bdrv_is_allocated_above(bs, NULL, false, offset, num,
+                                          &num);
+            if (ret < 0) {
+                return ret;
+            }
+            flags = ret ? NBD_STATE_DEPTH_BACKING : NBD_STATE_DEPTH_UNALLOC;
+        }
+
+        if (nbd_extent_array_add(ea, num, flags) < 0) {
+            return 0;
+        }
+
+        offset += num;
+        bytes -= num;
+    }
+
+    return 0;
+}
+
 /*
  * nbd_co_send_extents
  *
@@ -2000,7 +2055,11 @@ static int nbd_co_send_block_status(NBDClient *client, 
uint64_t handle,
     unsigned int nb_extents = dont_fragment ? 1 : NBD_MAX_BLOCK_STATUS_EXTENTS;
     g_autoptr(NBDExtentArray) ea = nbd_extent_array_new(nb_extents);

-    ret = blockstatus_to_extents(bs, offset, length, ea);
+    if (context_id == NBD_META_ID_BASE_ALLOCATION) {
+        ret = blockstatus_to_extents(bs, offset, length, ea);
+    } else {
+        ret = blockalloc_to_extents(bs, offset, length, ea);
+    }
     if (ret < 0) {
         return nbd_co_send_structured_error(
                 client, handle, -ret, "can't get block status", errp);
@@ -2335,16 +2394,20 @@ static coroutine_fn int nbd_handle_request(NBDClient 
*client,
         }
         if (client->export_meta.valid &&
             (client->export_meta.base_allocation ||
+             client->export_meta.allocation_depth ||
              client->export_meta.bitmap))
         {
             bool dont_fragment = request->flags & NBD_CMD_FLAG_REQ_ONE;
+            int contexts_remaining = client->export_meta.base_allocation +
+                client->export_meta.allocation_depth +
+                client->export_meta.bitmap;

             if (client->export_meta.base_allocation) {
                 ret = nbd_co_send_block_status(client, request->handle,
                                                blk_bs(exp->common.blk),
                                                request->from,
                                                request->len, dont_fragment,
-                                               !client->export_meta.bitmap,
+                                               !--contexts_remaining,
                                                NBD_META_ID_BASE_ALLOCATION,
                                                errp);
                 if (ret < 0) {
@@ -2352,17 +2415,32 @@ static coroutine_fn int nbd_handle_request(NBDClient 
*client,
                 }
             }

+            if (client->export_meta.allocation_depth) {
+                ret = nbd_co_send_block_status(client, request->handle,
+                                               blk_bs(exp->common.blk),
+                                               request->from, request->len,
+                                               dont_fragment,
+                                               !--contexts_remaining,
+                                               NBD_META_ID_ALLOCATION_DEPTH,
+                                               errp);
+                if (ret < 0) {
+                    return ret;
+                }
+            }
+
             if (client->export_meta.bitmap) {
                 ret = nbd_co_send_bitmap(client, request->handle,
                                          client->exp->export_bitmap,
                                          request->from, request->len,
-                                         dont_fragment,
-                                         true, NBD_META_ID_DIRTY_BITMAP, errp);
+                                         dont_fragment, !--contexts_remaining,
+                                         NBD_META_ID_DIRTY_BITMAP, errp);
                 if (ret < 0) {
                     return ret;
                 }
             }

+            assert(!contexts_remaining);
+
             return 0;
         } else {
             return nbd_send_generic_reply(client, request->handle, -EINVAL,
-- 
2.28.0