From d75cbb5e68f5a7d64248ba89399a7f2073231de5 Mon Sep 17 00:00:00 2001
From: Peter Lieven <pl@kamp.de>
Date: Thu, 24 Oct 2013 12:07:03 +0200
Subject: [PATCH] block: introduce bdrv_make_zero

this patch adds a call to completely zero out a block device.
the operation is sped up by checking the block status and
only writing zeroes to the device if they currently do not
return zeroes. optionally the zero writing can be sped up
by setting the flag BDRV_REQ_MAY_UNMAP to emulate the zero
write by unmapping if the driver supports it.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block.c               | 37 +++++++++++++++++++++++++++++++++++++
 include/block/block.h |  1 +
 2 files changed, 38 insertions(+)

diff --git a/block.c b/block.c
index df5a603686..c390f20042 100644
--- a/block.c
+++ b/block.c
@@ -2399,6 +2399,43 @@ int bdrv_write_zeroes(BlockDriverState *bs, int64_t sector_num,
                       BDRV_REQ_ZERO_WRITE | flags);
 }
 
+/*
+ * Completely zero out a block device with the help of bdrv_write_zeroes.
+ * The operation is sped up by checking the block status and only writing
+ * zeroes to the device if they currently do not return zeroes. Optional
+ * flags are passed through to bdrv_write_zeroes (e.g. BDRV_REQ_MAY_UNMAP).
+ *
+ * Returns < 0 on error, 0 on success. For error codes see bdrv_write().
+ */
+int bdrv_make_zero(BlockDriverState *bs, BdrvRequestFlags flags)
+{
+    int64_t target_size = bdrv_getlength(bs) / BDRV_SECTOR_SIZE;
+    int64_t ret, nb_sectors, sector_num = 0;
+    int n;
+
+    for (;;) {
+        nb_sectors = target_size - sector_num;
+        if (nb_sectors <= 0) {
+            return 0;
+        }
+        if (nb_sectors > INT_MAX) {
+            nb_sectors = INT_MAX;
+        }
+        ret = bdrv_get_block_status(bs, sector_num, nb_sectors, &n);
+        if (ret & BDRV_BLOCK_ZERO) {
+            sector_num += n;
+            continue;
+        }
+        ret = bdrv_write_zeroes(bs, sector_num, n, flags);
+        if (ret < 0) {
+            error_report("error writing zeroes at sector %" PRId64 ": %s",
+                         sector_num, strerror(-ret));
+            return ret;
+        }
+        sector_num += n;
+    }
+}
+
 int bdrv_pread(BlockDriverState *bs, int64_t offset,
                void *buf, int count1)
 {
diff --git a/include/block/block.h b/include/block/block.h
index 803c5caceb..4d9e67c7eb 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -216,6 +216,7 @@ int bdrv_write(BlockDriverState *bs, int64_t sector_num,
                const uint8_t *buf, int nb_sectors);
 int bdrv_write_zeroes(BlockDriverState *bs, int64_t sector_num,
                int nb_sectors, BdrvRequestFlags flags);
+int bdrv_make_zero(BlockDriverState *bs, BdrvRequestFlags flags);
 int bdrv_writev(BlockDriverState *bs, int64_t sector_num, QEMUIOVector *qiov);
 int bdrv_pread(BlockDriverState *bs, int64_t offset,
                void *buf, int count);