[PATCH v4 1/4] block: add max_pwrite_zeroes_fast to BlockLimits

[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). To drop the restriction we
need new max_pwrite_zeroes_fast.

Default value of new max_pwrite_zeroes_fast is zero and it means
use max_pwrite_zeroes. So this commit semantically changes nothing.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block_int.h |  8 ++++++++
 block/io.c                | 17 ++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 791de6a59c..277e32fe31 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -626,6 +626,14 @@ 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 zeroed at once if flag
+     * BDRV_REQ_NO_FALLBACK specified. Must be multiple of
+     * pwrite_zeroes_alignment.
+     * If 0, max_pwrite_zeroes is used for no-fallback case.
+     */
+    int64_t max_pwrite_zeroes_fast;
+
     /* 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/io.c b/block/io.c
index df8f2a98d4..0af62a53fd 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1774,12 +1774,13 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
     bool need_flush = false;
     int head = 0;
     int tail = 0;
-
-    int max_write_zeroes = MIN_NON_ZERO(bs->bl.max_pwrite_zeroes, INT_MAX);
+    int max_write_zeroes;
     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);
 
+    assert(alignment % bs->bl.request_alignment == 0);
+
     if (!drv) {
         return -ENOMEDIUM;
     }
@@ -1788,12 +1789,18 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
         return -ENOTSUP;
     }
 
-    assert(alignment % bs->bl.request_alignment == 0);
-    head = offset % alignment;
-    tail = (offset + bytes) % alignment;
+    if ((flags & BDRV_REQ_NO_FALLBACK) && bs->bl.max_pwrite_zeroes_fast) {
+        max_write_zeroes = bs->bl.max_pwrite_zeroes_fast;
+    } else {
+        max_write_zeroes = bs->bl.max_pwrite_zeroes;
+    }
+    max_write_zeroes = MIN_NON_ZERO(max_write_zeroes, INT_MAX);
     max_write_zeroes = QEMU_ALIGN_DOWN(max_write_zeroes, alignment);
     assert(max_write_zeroes >= bs->bl.request_alignment);
 
+    head = offset % alignment;
+    tail = (offset + bytes) % alignment;
+
     while (bytes > 0 && !ret) {
         int num = bytes;
 
-- 
2.21.0