[Qemu-devel] [PATCH] [RESEND] Add host_device support to qemu-img.

[Nolan <nolan@sigbus.net>]

This patch allows the use a host_device as the destination for "qemu-img
convert".

I added a ->bdrv_create function host_device.  It merely verifies that
the device exists and is large enough.

A check is needed in the qemu-img convert loop to ensure that we write
out all 0 sectors to the host_device if they were allocated in the
source.

I also made the check against bdrv_is_allocated unconditional, since
there is no point in making the block backend write a bunch of zeros
just so that we can memcmp them immediately afterwards.

Finally, there is an unrelated fix for a typo in the error message
printed if the destination device does not support ->bdrv_create.

Signed-off-by: Nolan Leake <nolan <at> sigbus.net>

Index: block-raw-posix.c
===================================================================
--- block-raw-posix.c	(revision 6963)
+++ block-raw-posix.c	(working copy)
@@ -1378,11 +1378,47 @@
 }
 #endif /* !linux && !FreeBSD */
 
+#if defined(__linux__) || defined(__FreeBSD__)
+static int hdev_create(const char *filename, int64_t total_size,
+                       const char *backing_file, int flags)
+{
+    int fd;
+    int ret = 0;
+    struct stat stat_buf;
+
+    if (flags || backing_file)
+        return -ENOTSUP;
+
+    fd = open(filename, O_WRONLY | O_BINARY);
+    if (fd < 0)
+        return -EIO;
+
+    if (fstat(fd, &stat_buf) < 0)
+        ret = -EIO;
+    else if (!S_ISBLK(stat_buf.st_mode))
+        ret = -EIO;
+    else if (lseek(fd, 0, SEEK_END) < total_size * 512)
+        ret = -ENOSPC;
+
+    close(fd);
+    return ret;
+}
+
+#else  /* !(linux || freebsd) */
+
+static int hdev_create(const char *filename, int64_t total_size,
+                       const char *backing_file, int flags)
+{
+    return -ENOTSUP;
+}
+#endif
+
 BlockDriver bdrv_host_device = {
     .format_name	= "host_device",
     .instance_size	= sizeof(BDRVRawState),
     .bdrv_open		= hdev_open,
     .bdrv_close		= raw_close,
+    .bdrv_create        = hdev_create,
     .bdrv_flush		= raw_flush,
 
 #ifdef CONFIG_AIO
Index: qemu-img.c
===================================================================
--- qemu-img.c	(revision 6963)
+++ qemu-img.c	(working copy)
@@ -493,7 +493,7 @@
     ret = bdrv_create(drv, out_filename, total_sectors, out_baseimg, flags);
     if (ret < 0) {
         if (ret == -ENOTSUP) {
-            error("Formatting not supported for file format '%s'", fmt);
+            error("Formatting not supported for file format '%s'", out_fmt);
         } else {
             error("Error while formatting '%s'", out_filename);
         }
@@ -592,19 +592,13 @@
             if (n > bs_offset + bs_sectors - sector_num)
                 n = bs_offset + bs_sectors - sector_num;
 
-            /* If the output image is being created as a copy on write image,
-               assume that sectors which are unallocated in the input image
-               are present in both the output's and input's base images (no
-               need to copy them). */
-            if (out_baseimg) {
-               if (!bdrv_is_allocated(bs[bs_i], sector_num - bs_offset, n, &n1)) {
-                  sector_num += n1;
-                  continue;
-               }
-               /* The next 'n1' sectors are allocated in the input image. Copy
-                  only those as they may be followed by unallocated sectors. */
-               n = n1;
+            if (!bdrv_is_allocated(bs[bs_i], sector_num - bs_offset, n, &n1)) {
+                sector_num += n1;
+                continue;
             }
+            /* The next 'n1' sectors are allocated in the input image. Copy
+               only those as they may be followed by unallocated sectors. */
+            n = n1;
 
             if (bdrv_read(bs[bs_i], sector_num - bs_offset, buf, n) < 0) 
                 error("error while reading");
@@ -615,8 +609,13 @@
             while (n > 0) {
                 /* If the output image is being created as a copy on write image,
                    copy all sectors even the ones containing only NUL bytes,
-                   because they may differ from the sectors in the base image. */
-                if (out_baseimg || is_allocated_sectors(buf1, n, &n1)) {
+                   because they may differ from the sectors in the base image.
+
+                   If the output is to a host device, we also write out
+                   sectors that are entirely 0, since whatever data was
+                   already there is garbage, not 0s. */
+                if (drv == &bdrv_host_device || out_baseimg ||
+                    is_allocated_sectors(buf1, n, &n1)) {
                     if (bdrv_write(out_bs, sector_num, buf1, n1) < 0)
                         error("error while writing");
                 }