[PATCH 3/5] block: add max_pwrite_zeroes_no_fallback to BlockLimits

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

NBD spec is updated, so 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). To drop the restriction we need new
max_pwrite_zeroes_no_fallback.

Default value of new max_pwrite_zeroes_no_fallback is zero and it means
no-restriction, so we are automatically done by this commit. Note that
nbd and blkdebug are the only drivers which in the same time define
max_pwrite_zeroes limit and support BDRV_REQ_NO_FALLBACK, so we need to
update only blkdebug.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block_int.h | 8 ++++++++
 block/blkdebug.c          | 7 ++++++-
 block/io.c                | 4 +++-
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 6f9fd5e20e..c167e887c6 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -618,6 +618,14 @@ typedef struct BlockLimits {
      * pwrite_zeroes_alignment. May be 0 if no inherent 32-bit limit */
     int32_t max_pwrite_zeroes;
 
+    /*
+     * Maximum number of bytes that can zeroized at once if flag
+     * BDRV_REQ_NO_FALLBACK specified (since it is signed, it must be < 2G, if
+     * set). Must be multiple of pwrite_zeroes_alignment. May be 0 if no
+     * inherent 32-bit limit.
+     */
+    int32_t max_pwrite_zeroes_no_fallback;
+
     /* Optimal alignment for write zeroes requests in bytes. A power
      * of 2 is best but not mandatory.  Must be a multiple of
      * bl.request_alignment, and must be less than max_pwrite_zeroes
diff --git a/block/blkdebug.c b/block/blkdebug.c
index af44aa973f..7627fbcb3b 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -692,7 +692,11 @@ static int coroutine_fn blkdebug_co_pwrite_zeroes(BlockDriverState *bs,
     }
     assert(QEMU_IS_ALIGNED(offset, align));
     assert(QEMU_IS_ALIGNED(bytes, align));
-    if (bs->bl.max_pwrite_zeroes) {
+    if ((flags & BDRV_REQ_NO_FALLBACK) &&
+        bs->bl.max_pwrite_zeroes_no_fallback)
+    {
+        assert(bytes <= bs->bl.max_pwrite_zeroes_no_fallback);
+    } else if (bs->bl.max_pwrite_zeroes) {
         assert(bytes <= bs->bl.max_pwrite_zeroes);
     }
 
@@ -977,6 +981,7 @@ static void blkdebug_refresh_limits(BlockDriverState *bs, Error **errp)
     }
     if (s->max_write_zero) {
         bs->bl.max_pwrite_zeroes = s->max_write_zero;
+        bs->bl.max_pwrite_zeroes_no_fallback = s->max_write_zero;
     }
     if (s->opt_discard) {
         bs->bl.pdiscard_alignment = s->opt_discard;
diff --git a/block/io.c b/block/io.c
index 7e4cb74cf4..75fd5600c2 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1752,7 +1752,9 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
     int head = 0;
     int tail = 0;
 
-    int max_write_zeroes = MIN_NON_ZERO(bs->bl.max_pwrite_zeroes, INT_MAX);
+    int max_write_zeroes = MIN_NON_ZERO((flags & BDRV_REQ_NO_FALLBACK) ?
+                                        bs->bl.max_pwrite_zeroes_no_fallback :
+                                        bs->bl.max_pwrite_zeroes, INT_MAX);
     int alignment = MAX(bs->bl.pwrite_zeroes_alignment,
                         bs->bl.request_alignment);
     int max_transfer = MIN_NON_ZERO(bs->bl.max_transfer, MAX_BOUNCE_BUFFER);
-- 
2.21.0