[PATCH v4 2/4] block/nbd: define new max_write_zero_fast limit

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

The NBD spec was recently updated to clarify that max_block doesn't
relate to NBD_CMD_WRITE_ZEROES with NBD_CMD_FLAG_FAST_ZERO (which
mirrors Qemu flag BDRV_REQ_NO_FALLBACK).

bs->bl.max_write_zero_fast is zero by default which means using
max_pwrite_zeroes. Update nbd driver to allow larger requests with
BDRV_REQ_NO_FALLBACK.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/nbd.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block/nbd.c b/block/nbd.c
index 4ac23c8f62..b0584cf68d 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -1956,6 +1956,7 @@ static void nbd_refresh_limits(BlockDriverState *bs, Error **errp)
 
     bs->bl.request_alignment = min;
     bs->bl.max_pdiscard = QEMU_ALIGN_DOWN(INT_MAX, min);
+    bs->bl.max_pwrite_zeroes_fast = bs->bl.max_pdiscard;
     bs->bl.max_pwrite_zeroes = max;
     bs->bl.max_transfer = max;
 
-- 
2.21.0