[Qemu-devel] [PATCH 4/5] nbd: Implement server use of NBD FAST_ZERO

[Eric Blake <eblake@redhat.com>]

The server side is fairly straightforward: we can always advertise
support for detection of fast zero, and implement it by mapping the
request to the block layer BDRV_REQ_NO_FALLBACK.

Signed-off-by: Eric Blake <eblake@redhat.com>

---

Again, this may be worth merging with the previous patch.
---
 nbd/server.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 981bc3cb1151..3c0799eda87f 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1514,7 +1514,8 @@ NBDExport *nbd_export_new(BlockDriverState *bs, uint64_t dev_offset,
             exp->nbdflags |= NBD_FLAG_CAN_MULTI_CONN;
         }
     } else {
-        exp->nbdflags |= NBD_FLAG_SEND_TRIM | NBD_FLAG_SEND_WRITE_ZEROES;
+        exp->nbdflags |= (NBD_FLAG_SEND_TRIM | NBD_FLAG_SEND_WRITE_ZEROES |
+                          NBD_FLAG_SEND_FAST_ZERO);
     }
     assert(size <= INT64_MAX - dev_offset);
     exp->size = QEMU_ALIGN_DOWN(size, BDRV_SECTOR_SIZE);
@@ -2167,7 +2168,7 @@ static int nbd_co_receive_request(NBDRequestData *req, NBDRequest *request,
     if (request->type == NBD_CMD_READ && client->structured_reply) {
         valid_flags |= NBD_CMD_FLAG_DF;
     } else if (request->type == NBD_CMD_WRITE_ZEROES) {
-        valid_flags |= NBD_CMD_FLAG_NO_HOLE;
+        valid_flags |= NBD_CMD_FLAG_NO_HOLE | NBD_CMD_FLAG_FAST_ZERO;
     } else if (request->type == NBD_CMD_BLOCK_STATUS) {
         valid_flags |= NBD_CMD_FLAG_REQ_ONE;
     }
@@ -2306,6 +2307,9 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
         if (!(request->flags & NBD_CMD_FLAG_NO_HOLE)) {
             flags |= BDRV_REQ_MAY_UNMAP;
         }
+        if (request->flags & NBD_CMD_FLAG_FAST_ZERO) {
+            flags |= BDRV_REQ_NO_FALLBACK;
+        }
         ret = blk_pwrite_zeroes(exp->blk, request->from + exp->dev_offset,
                                 request->len, flags);
         return nbd_send_generic_reply(client, request->handle, ret,
-- 
2.21.0