nbd: Implement NBD_CMD_WRITE_ZEROES on client

Upstream NBD protocol recently added the ability to efficiently
write zeroes without having to send the zeroes over the wire,
along with a flag to control whether the client wants a hole.

The generic block code takes care of falling back to the obvious
write of lots of zeroes if we return -ENOTSUP because the server
does not have WRITE_ZEROES.

Ideally, since NBD_CMD_WRITE_ZEROES does not involve any data
over the wire, we want to support transactions that are much
larger than the normal 32M limit imposed on NBD_CMD_WRITE.  But
the server may still have a limit smaller than UINT_MAX, so
until experimental NBD protocol additions for advertising various
command sizes is finalized (see [1], [2]), for now we just stick to
the same limits as normal writes.

[1] https://github.com/yoe/nbd/blob/extension-info/doc/proto.md
[2] https://sourceforge.net/p/nbd/mailman/message/35081223/Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <1476469998-28592-17-git-send-email-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

block/nbd-client.c

@@ -275,6 +275,41 @@ int nbd_client_co_pwritev(BlockDriverState *bs, uint64_t offset,
     return -reply.error;
 }
 
+int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
+                                int count, BdrvRequestFlags flags)
+{
+    ssize_t ret;
+    NBDClientSession *client = nbd_get_client_session(bs);
+    NBDRequest request = {
+        .type = NBD_CMD_WRITE_ZEROES,
+        .from = offset,
+        .len = count,
+    };
+    NBDReply reply;
+
+    if (!(client->nbdflags & NBD_FLAG_SEND_WRITE_ZEROES)) {
+        return -ENOTSUP;
+    }
+
+    if (flags & BDRV_REQ_FUA) {
+        assert(client->nbdflags & NBD_FLAG_SEND_FUA);
+        request.flags |= NBD_CMD_FLAG_FUA;
+    }
+    if (!(flags & BDRV_REQ_MAY_UNMAP)) {
+        request.flags |= NBD_CMD_FLAG_NO_HOLE;
+    }
+
+    nbd_coroutine_start(client, &request);
+    ret = nbd_co_send_request(bs, &request, NULL);
+    if (ret < 0) {
+        reply.error = -ret;
+    } else {
+        nbd_co_receive_reply(client, &request, &reply, NULL);
+    }
+    nbd_coroutine_end(client, &request);
+    return -reply.error;
+}
+
 int nbd_client_co_flush(BlockDriverState *bs)
 {
     NBDClientSession *client = nbd_get_client_session(bs);

block/nbd-client.h

@@ -48,6 +48,8 @@ int nbd_client_co_pdiscard(BlockDriverState *bs, int64_t offset, int count);
 int nbd_client_co_flush(BlockDriverState *bs);
 int nbd_client_co_pwritev(BlockDriverState *bs, uint64_t offset,
                           uint64_t bytes, QEMUIOVector *qiov, int flags);
+int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
+                                int count, BdrvRequestFlags flags);
 int nbd_client_co_preadv(BlockDriverState *bs, uint64_t offset,
                          uint64_t bytes, QEMUIOVector *qiov, int flags);
 

block/nbd.c

@@ -466,6 +466,7 @@ static int nbd_co_flush(BlockDriverState *bs)
 static void nbd_refresh_limits(BlockDriverState *bs, Error **errp)
 {
     bs->bl.max_pdiscard = NBD_MAX_BUFFER_SIZE;
+    bs->bl.max_pwrite_zeroes = NBD_MAX_BUFFER_SIZE;
     bs->bl.max_transfer = NBD_MAX_BUFFER_SIZE;
 }
 
@@ -558,6 +559,7 @@ static BlockDriver bdrv_nbd = {
     .bdrv_file_open             = nbd_open,
     .bdrv_co_preadv             = nbd_client_co_preadv,
     .bdrv_co_pwritev            = nbd_client_co_pwritev,
+    .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
     .bdrv_close                 = nbd_close,
     .bdrv_co_flush_to_os        = nbd_co_flush,
     .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
@@ -576,6 +578,7 @@ static BlockDriver bdrv_nbd_tcp = {
     .bdrv_file_open             = nbd_open,
     .bdrv_co_preadv             = nbd_client_co_preadv,
     .bdrv_co_pwritev            = nbd_client_co_pwritev,
+    .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
     .bdrv_close                 = nbd_close,
     .bdrv_co_flush_to_os        = nbd_co_flush,
     .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
@@ -594,6 +597,7 @@ static BlockDriver bdrv_nbd_unix = {
     .bdrv_file_open             = nbd_open,
     .bdrv_co_preadv             = nbd_client_co_preadv,
     .bdrv_co_pwritev            = nbd_client_co_pwritev,
+    .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
     .bdrv_close                 = nbd_close,
     .bdrv_co_flush_to_os        = nbd_co_flush,
     .bdrv_co_pdiscard           = nbd_client_co_pdiscard,