nbd: Add 'qemu-nbd -A' to expose allocation depth

Allow the server to expose an additional metacontext to be requested
by savvy clients.  qemu-nbd adds a new option -A to expose the
qemu:allocation-depth metacontext through NBD_CMD_BLOCK_STATUS; this
can also be set via QMP when using block-export-add.

qemu as client is hacked into viewing the key aspects of this new
context by abusing the already-experimental x-dirty-bitmap option to
collapse all depths greater than 2, which results in a tri-state value
visible in the output of 'qemu-img map --output=json' (yes, that means
x-dirty-bitmap is now a bit of a misnomer, but I didn't feel like
renaming it as it would introduce a needless break of back-compat,
even though we make no compat guarantees with x- members):

unallocated (depth 0) => "zero":false, "data":true
local (depth 1)       => "zero":false, "data":false
backing (depth 2+)    => "zero":true,  "data":true

libnbd as client is probably a nicer way to get at the information
without having to decipher such hacks in qemu as client. ;)

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20201027050556.269064-11-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

1 files changed, 21 insertions, 5 deletions

diff --git a/block/nbd.c b/block/nbd.c
index 4548046cd7..42536702b6 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -135,6 +135,7 @@ typedef struct BDRVNBDState {
     QCryptoTLSCreds *tlscreds;
     const char *hostname;
     char *x_dirty_bitmap;
+    bool alloc_depth;
 
     bool wait_connect;
     NBDConnectThread *connect_thread;
@@ -961,6 +962,16 @@ static int nbd_parse_blockstatus_payload(BDRVNBDState *s,
         trace_nbd_parse_blockstatus_compliance("extent length too large");
     }
 
+    /*
+     * HACK: if we are using x-dirty-bitmaps to access
+     * qemu:allocation-depth, treat all depths > 2 the same as 2,
+     * since nbd_client_co_block_status is only expecting the low two
+     * bits to be set.
+     */
+    if (s->alloc_depth && extent->flags > 2) {
+        extent->flags = 2;
+    }
+
     return 0;
 }
 
@@ -1795,11 +1806,16 @@ static int nbd_client_handshake(BlockDriverState *bs, QIOChannelSocket *sioc,
         s->sioc = NULL;
         return ret;
     }
-    if (s->x_dirty_bitmap && !s->info.base_allocation) {
-        error_setg(errp, "requested x-dirty-bitmap %s not found",
-                   s->x_dirty_bitmap);
-        ret = -EINVAL;
-        goto fail;
+    if (s->x_dirty_bitmap) {
+        if (!s->info.base_allocation) {
+            error_setg(errp, "requested x-dirty-bitmap %s not found",
+                       s->x_dirty_bitmap);
+            ret = -EINVAL;
+            goto fail;
+        }
+        if (strcmp(s->x_dirty_bitmap, "qemu:allocation-depth") == 0) {
+            s->alloc_depth = true;
+        }
     }
     if (s->info.flags & NBD_FLAG_READ_ONLY) {
         ret = bdrv_apply_auto_read_only(bs, "NBD export is read-only", errp);