1 files changed, 46 insertions, 31 deletions

diff --git a/qemu-img.c b/qemu-img.c
index e6ad5978e0..28fba1e7a7 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -37,6 +37,7 @@
 #include "qemu/option.h"
 #include "qemu/error-report.h"
 #include "qemu/log.h"
+#include "qemu/units.h"
 #include "qom/object_interfaces.h"
 #include "sysemu/sysemu.h"
 #include "sysemu/block-backend.h"
@@ -1216,7 +1217,7 @@ static int compare_buffers(const uint8_t *buf1, const uint8_t *buf2,
     return res;
 }
 
-#define IO_BUF_SIZE (2 * 1024 * 1024)
+#define IO_BUF_SIZE (2 * MiB)
 
 /*
  * Check if passed sectors are empty (not allocated or contain only 0 bytes)
@@ -2960,7 +2961,7 @@ static int img_map(int argc, char **argv)
         int64_t n;
 
         /* Probe up to 1 GiB at a time.  */
-        n = MIN(1 << 30, length - offset);
+        n = MIN(1 * GiB, length - offset);
         ret = get_block_status(bs, offset, n, &next);
 
         if (ret < 0) {
@@ -3311,26 +3312,30 @@ static int img_rebase(int argc, char **argv)
         char backing_name[PATH_MAX];
         QDict *options = NULL;
 
-        if (bs->backing_format[0] != '\0') {
-            options = qdict_new();
-            qdict_put_str(options, "driver", bs->backing_format);
-        }
-
-        if (force_share) {
-            if (!options) {
+        if (bs->backing) {
+            if (bs->backing_format[0] != '\0') {
                 options = qdict_new();
+                qdict_put_str(options, "driver", bs->backing_format);
             }
-            qdict_put_bool(options, BDRV_OPT_FORCE_SHARE, true);
-        }
-        bdrv_get_backing_filename(bs, backing_name, sizeof(backing_name));
-        blk_old_backing = blk_new_open(backing_name, NULL,
-                                       options, src_flags, &local_err);
-        if (!blk_old_backing) {
-            error_reportf_err(local_err,
-                              "Could not open old backing file '%s': ",
-                              backing_name);
-            ret = -1;
-            goto out;
+
+            if (force_share) {
+                if (!options) {
+                    options = qdict_new();
+                }
+                qdict_put_bool(options, BDRV_OPT_FORCE_SHARE, true);
+            }
+            bdrv_get_backing_filename(bs, backing_name, sizeof(backing_name));
+            blk_old_backing = blk_new_open(backing_name, NULL,
+                                           options, src_flags, &local_err);
+            if (!blk_old_backing) {
+                error_reportf_err(local_err,
+                                  "Could not open old backing file '%s': ",
+                                  backing_name);
+                ret = -1;
+                goto out;
+            }
+        } else {
+            blk_old_backing = NULL;
         }
 
         if (out_baseimg[0]) {
@@ -3383,7 +3388,7 @@ static int img_rebase(int argc, char **argv)
      */
     if (!unsafe) {
         int64_t size;
-        int64_t old_backing_size;
+        int64_t old_backing_size = 0;
         int64_t new_backing_size = 0;
         uint64_t offset;
         int64_t n;
@@ -3399,15 +3404,18 @@ static int img_rebase(int argc, char **argv)
             ret = -1;
             goto out;
         }
-        old_backing_size = blk_getlength(blk_old_backing);
-        if (old_backing_size < 0) {
-            char backing_name[PATH_MAX];
+        if (blk_old_backing) {
+            old_backing_size = blk_getlength(blk_old_backing);
+            if (old_backing_size < 0) {
+                char backing_name[PATH_MAX];
 
-            bdrv_get_backing_filename(bs, backing_name, sizeof(backing_name));
-            error_report("Could not get size of '%s': %s",
-                         backing_name, strerror(-old_backing_size));
-            ret = -1;
-            goto out;
+                bdrv_get_backing_filename(bs, backing_name,
+                                          sizeof(backing_name));
+                error_report("Could not get size of '%s': %s",
+                             backing_name, strerror(-old_backing_size));
+                ret = -1;
+                goto out;
+            }
         }
         if (blk_new_backing) {
             new_backing_size = blk_getlength(blk_new_backing);
@@ -3424,6 +3432,8 @@ static int img_rebase(int argc, char **argv)
         }
 
         for (offset = 0; offset < size; offset += n) {
+            bool buf_old_is_zero = false;
+
             /* How many bytes can we handle with the next read? */
             n = MIN(IO_BUF_SIZE, size - offset);
 
@@ -3444,6 +3454,7 @@ static int img_rebase(int argc, char **argv)
              */
             if (offset >= old_backing_size) {
                 memset(buf_old, 0, n);
+                buf_old_is_zero = true;
             } else {
                 if (offset + n > old_backing_size) {
                     n = old_backing_size - offset;
@@ -3479,8 +3490,12 @@ static int img_rebase(int argc, char **argv)
                 if (compare_buffers(buf_old + written, buf_new + written,
                                     n - written, &pnum))
                 {
-                    ret = blk_pwrite(blk, offset + written,
-                                     buf_old + written, pnum, 0);
+                    if (buf_old_is_zero) {
+                        ret = blk_pwrite_zeroes(blk, offset + written, pnum, 0);
+                    } else {
+                        ret = blk_pwrite(blk, offset + written,
+                                         buf_old + written, pnum, 0);
+                    }
                     if (ret < 0) {
                         error_report("Error while writing to COW image: %s",
                             strerror(-ret));