[PATCH v4 4/4] block/io: auto-no-fallback for write-zeroes

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

When BDRV_REQ_NO_FALLBACK is supported, the NBD driver supports a
larger request size.  Add code to try large zero requests with a
NO_FALLBACK request prior to having to split a request into chunks
according to max_pwrite_zeroes.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/io.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/block/io.c b/block/io.c
index 3fae97da2d..ad219cb220 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1778,6 +1778,7 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
     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);
+    bool auto_no_fallback = false;
 
     assert(alignment % bs->bl.request_alignment == 0);
 
@@ -1785,6 +1786,16 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
         return -ENOMEDIUM;
     }
 
+    if (!(flags & BDRV_REQ_NO_FALLBACK) &&
+        (bs->supported_zero_flags & BDRV_REQ_NO_FALLBACK) &&
+        bs->bl.max_pwrite_zeroes && bs->bl.max_pwrite_zeroes < bytes &&
+        bs->bl.max_pwrite_zeroes < bs->bl.max_pwrite_zeroes_fast)
+    {
+        assert(drv->bdrv_co_pwrite_zeroes);
+        flags |= BDRV_REQ_NO_FALLBACK;
+        auto_no_fallback = true;
+    }
+
     if ((flags & ~bs->supported_zero_flags) & BDRV_REQ_NO_FALLBACK) {
         return -ENOTSUP;
     }
@@ -1829,6 +1840,14 @@ static int coroutine_fn bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
         if (drv->bdrv_co_pwrite_zeroes) {
             ret = drv->bdrv_co_pwrite_zeroes(bs, offset, num,
                                              flags & bs->supported_zero_flags);
+            if (ret == -ENOTSUP && auto_no_fallback) {
+                auto_no_fallback = false;
+                flags &= ~BDRV_REQ_NO_FALLBACK;
+                max_write_zeroes =
+                    QEMU_ALIGN_DOWN(MIN_NON_ZERO(bs->bl.max_pwrite_zeroes,
+                                                 INT_MAX), alignment);
+                continue;
+            }
             if (ret != -ENOTSUP && (flags & BDRV_REQ_FUA) &&
                 !(bs->supported_zero_flags & BDRV_REQ_FUA)) {
                 need_flush = true;
-- 
2.21.0