nbd: Implement client use of NBD FAST_ZERO

The client side is fairly straightforward: if the server advertised
fast zero support, then we can map that to BDRV_REQ_NO_FALLBACK
support.  A server that advertises FAST_ZERO but not WRITE_ZEROES
is technically broken, but we can ignore that situation as it does
not change our behavior.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190823143726.27062-4-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

diff --git a/block/nbd.c b/block/nbd.c
index c4c91a158602ccf89d5334324503d77af33c2bfe..813c40d8f067b84bd1ff6c43b762d0f0cac67ce9 100644 (file)
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -1044,6 +1044,10 @@ static int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
     if (!(flags & BDRV_REQ_MAY_UNMAP)) {
         request.flags |= NBD_CMD_FLAG_NO_HOLE;
     }
+    if (flags & BDRV_REQ_NO_FALLBACK) {
+        assert(s->info.flags & NBD_FLAG_SEND_FAST_ZERO);
+        request.flags |= NBD_CMD_FLAG_FAST_ZERO;
+    }
 
     if (!bytes) {
         return 0;
@@ -1239,6 +1243,9 @@ static int nbd_client_connect(BlockDriverState *bs, Error **errp)
     }
     if (s->info.flags & NBD_FLAG_SEND_WRITE_ZEROES) {
         bs->supported_zero_flags |= BDRV_REQ_MAY_UNMAP;
+        if (s->info.flags & NBD_FLAG_SEND_FAST_ZERO) {
+            bs->supported_zero_flags |= BDRV_REQ_NO_FALLBACK;
+        }
     }
 
     s->sioc = sioc;