[PATCH v2 2/3] block: use pwrite_zeroes_alignment when writing first sector

[Stefan Hajnoczi <stefanha@redhat.com>]

Since commit 5634622bcb33 ("file-posix: allow BLKZEROOUT with -t
writeback"), qemu-img create errors out on a Linux loop block device
with a 4 KB sector size:

  # dd if=/dev/zero of=blockfile bs=1M count=1024
  # losetup --sector-size 4096 /dev/loop0 blockfile
  # qemu-img create -f raw /dev/loop0 1G
  Formatting '/dev/loop0', fmt=raw size=1073741824
  qemu-img: /dev/loop0: Failed to clear the new image's first sector: Invalid argument

Use the pwrite_zeroes_alignment block limit to avoid misaligned
fallocate(2) or ioctl(BLKZEROOUT) in the block/file-posix.c block
driver.

Fixes: 5634622bcb33 ("file-posix: allow BLKZEROOUT with -t writeback")
Reported-by: Jean-Louis Dupond <jean-louis@dupond.be>
Buglink: https://gitlab.com/qemu-project/qemu/-/issues/3127
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/system/block-backend-io.h |  1 +
 block.c                           |  3 ++-
 block/block-backend.c             | 11 +++++++++++
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/system/block-backend-io.h b/include/system/block-backend-io.h
index ba8dfcc7d0..6d5ac476fc 100644
--- a/include/system/block-backend-io.h
+++ b/include/system/block-backend-io.h
@@ -116,6 +116,7 @@ BlockAIOCB *blk_abort_aio_request(BlockBackend *blk,
                                   void *opaque, int ret);
 
 uint32_t blk_get_request_alignment(BlockBackend *blk);
+uint32_t blk_get_pwrite_zeroes_alignment(BlockBackend *blk);
 uint32_t blk_get_max_transfer(BlockBackend *blk);
 uint64_t blk_get_max_hw_transfer(BlockBackend *blk);
 
diff --git a/block.c b/block.c
index 8848e9a7ed..be77e03904 100644
--- a/block.c
+++ b/block.c
@@ -606,12 +606,13 @@ create_file_fallback_zero_first_sector(BlockBackend *blk,
                                        int64_t current_size,
                                        Error **errp)
 {
+    uint32_t alignment = blk_get_pwrite_zeroes_alignment(blk);
     int64_t bytes_to_clear;
     int ret;
 
     GLOBAL_STATE_CODE();
 
-    bytes_to_clear = MIN(current_size, BDRV_SECTOR_SIZE);
+    bytes_to_clear = MIN(current_size, MAX(BDRV_SECTOR_SIZE, alignment));
     if (bytes_to_clear) {
         ret = blk_co_pwrite_zeroes(blk, 0, bytes_to_clear, BDRV_REQ_MAY_UNMAP);
         if (ret < 0) {
diff --git a/block/block-backend.c b/block/block-backend.c
index f8d6ba65c1..239d6eca37 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2305,6 +2305,17 @@ uint32_t blk_get_request_alignment(BlockBackend *blk)
     return bs ? bs->bl.request_alignment : BDRV_SECTOR_SIZE;
 }
 
+/* Returns the optimal write zeroes alignment, in bytes; guaranteed nonzero */
+uint32_t blk_get_pwrite_zeroes_alignment(BlockBackend *blk)
+{
+    BlockDriverState *bs = blk_bs(blk);
+    IO_CODE();
+    if (!bs) {
+        return BDRV_SECTOR_SIZE;
+    }
+    return bs->bl.pwrite_zeroes_alignment ?: bs->bl.request_alignment;
+}
+
 /* Returns the maximum hardware transfer length, in bytes; guaranteed nonzero */
 uint64_t blk_get_max_hw_transfer(BlockBackend *blk)
 {
-- 
2.51.0