[PATCH v5 4/9] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate

[Kevin Wolf <kwolf@redhat.com>]

If BDRV_REQ_ZERO_WRITE is set and we're extending the image, calling
qcow2_cluster_zeroize() with flags=0 does the right thing: It doesn't
undo any previous preallocation, but just adds the zero flag to all
relevant L2 entries. If an external data file is in use, a write_zeroes
request to the data file is made instead.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/block/qcow2.c b/block/qcow2.c
index 9cfbdfc939..bd632405d1 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1726,6 +1726,7 @@ static int coroutine_fn qcow2_do_open(BlockDriverState *bs, QDict *options,
 
     bs->supported_zero_flags = header.version >= 3 ?
                                BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK : 0;
+    bs->supported_truncate_flags = BDRV_REQ_ZERO_WRITE;
 
     /* Repair image if dirty */
     if (!(flags & (BDRV_O_CHECK | BDRV_O_INACTIVE)) && !bs->read_only &&
@@ -4214,6 +4215,35 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
         g_assert_not_reached();
     }
 
+    if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
+        uint64_t zero_start = QEMU_ALIGN_UP(old_length, s->cluster_size);
+        uint64_t zero_end = QEMU_ALIGN_UP(offset, s->cluster_size);
+
+        /* Use zero clusters as much as we can */
+        ret = qcow2_cluster_zeroize(bs, zero_start, zero_end - zero_start, 0);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "Failed to zero out the new area");
+            goto fail;
+        }
+
+        /* Write explicit zeros for the unaligned head */
+        if (zero_start > old_length) {
+            uint8_t *buf = qemu_blockalign0(bs, s->cluster_size);
+            QEMUIOVector qiov;
+            qemu_iovec_init_buf(&qiov, buf, zero_start - old_length);
+
+            qemu_co_mutex_unlock(&s->lock);
+            ret = qcow2_co_pwritev_part(bs, old_length, qiov.size, &qiov, 0, 0);
+            qemu_co_mutex_lock(&s->lock);
+
+            qemu_vfree(buf);
+            if (ret < 0) {
+                error_setg_errno(errp, -ret, "Failed to zero out the new area");
+                goto fail;
+            }
+        }
+    }
+
     if (prealloc != PREALLOC_MODE_OFF) {
         /* Flush metadata before actually changing the image size */
         ret = qcow2_write_caches(bs);
-- 
2.25.3