[Qemu-devel] [PATCH v2 3/4] block/vpc: give option to force the current_size field in .bdrv_create

[Jeff Cody <jcody@redhat.com>]

When QEMU creates a VHD image, it goes by the original spec,
calculating the current_size based on the nearest CHS geometry (with an
exception for disks > 127GB).

Apparently, Azure will only allow images that are sized to the nearest
MB, and the current_size as calculated from CHS cannot guarantee that.

Allow QEMU to create images similar to how Hyper-V creates images, by
setting current_size to the specified virtual disk size.  This
introduces an option, force_size, to be passed to the vpc format during
image creation, e.g.:

    qemu-img convert -f raw -o force_size -O vpc test.img test.vhd

When using the "force_size" option, the creator app field used by
QEMU will be "qem2" instead of "qemu", to indicate the difference.
In light of this, we also add parsing of the "qem2" field during
vpc_open.

Bug reference: https://bugs.launchpad.net/qemu/+bug/1490611

Signed-off-by: Jeff Cody <jcody@redhat.com>
---
 block/vpc.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/block/vpc.c b/block/vpc.c
index 54a38a7..8f84b17 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -49,6 +49,8 @@ enum vhd_type {
 #define VHD_MAX_SECTORS       (65535LL * 255 * 255)
 #define VHD_MAX_GEOMETRY      (65535LL *  16 * 255)
 
+#define VPC_OPT_FORCE_SIZE "force_size"
+
 // always big-endian
 typedef struct vhd_footer {
     char        creator[8]; // "conectix"
@@ -288,6 +290,7 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
      *  Known creator apps:
      *      'vpc '  :  CHS              Virtual PC (uses disk geometry)
      *      'qemu'  :  CHS              QEMU (uses disk geometry)
+     *      'qem2'  :  current_size     QEMU (uses current_size)
      *      'win '  :  current_size     Hyper-V
      *      'd2v '  :  current_size     Disk2vhd
      *
@@ -296,6 +299,7 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
      *  that have CHS geometry of the maximum size.
      */
     use_chs = (!!strncmp(footer->creator_app, "win ", 4) &&
+               !!strncmp(footer->creator_app, "qem2", 4) &&
                !!strncmp(footer->creator_app, "d2v ", 4)) || s->force_use_chs;
 
     if (!use_chs || bs->total_sectors == VHD_MAX_GEOMETRY || s->force_use_sz) {
@@ -850,6 +854,7 @@ static int vpc_create(const char *filename, QemuOpts *opts, Error **errp)
     int64_t total_size;
     int disk_type;
     int ret = -EIO;
+    bool force_size;
     Error *local_err = NULL;
     BlockDriverState *bs = NULL;
 
@@ -870,6 +875,8 @@ static int vpc_create(const char *filename, QemuOpts *opts, Error **errp)
         disk_type = VHD_DYNAMIC;
     }
 
+    force_size = qemu_opt_get_bool_del(opts, VPC_OPT_FORCE_SIZE, false);
+
     ret = bdrv_create_file(filename, opts, &local_err);
     if (ret < 0) {
         error_propagate(errp, local_err);
@@ -896,7 +903,8 @@ static int vpc_create(const char *filename, QemuOpts *opts, Error **errp)
         calculate_geometry(total_sectors + i, &cyls, &heads, &secs_per_cyl);
     }
 
-    if ((int64_t)cyls * heads * secs_per_cyl == VHD_MAX_GEOMETRY) {
+    if ((int64_t)cyls * heads * secs_per_cyl == VHD_MAX_GEOMETRY ||
+            force_size) {
         total_sectors = total_size / BDRV_SECTOR_SIZE;
         /* Allow a maximum disk size of approximately 2 TB */
         if (total_sectors > VHD_MAX_SECTORS) {
@@ -912,8 +920,11 @@ static int vpc_create(const char *filename, QemuOpts *opts, Error **errp)
     memset(buf, 0, 1024);
 
     memcpy(footer->creator, "conectix", 8);
-    /* TODO Check if "qemu" creator_app is ok for VPC */
-    memcpy(footer->creator_app, "qemu", 4);
+    if (force_size) {
+        memcpy(footer->creator_app, "qem2", 4);
+    } else {
+        memcpy(footer->creator_app, "qemu", 4);
+    }
     memcpy(footer->creator_os, "Wi2k", 4);
 
     footer->features = cpu_to_be32(0x02);
@@ -994,6 +1005,13 @@ static QemuOptsList vpc_create_opts = {
                 "Type of virtual hard disk format. Supported formats are "
                 "{dynamic (default) | fixed} "
         },
+        {
+            .name = VPC_OPT_FORCE_SIZE,
+            .type = QEMU_OPT_BOOL,
+            .help = "Force disk size calculation to use the actual size "
+                    "specified, rather than using the nearest CHS-based "
+                    "calculation"
+        },
         { /* end of list */ }
     }
 };
-- 
1.9.3