[Qemu-devel] block/vpc: Fix vhd extra sectors issue

[Lucian Petrut <petrutlucian94@gmail.com>]

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At the moment, qemu-img extends new image virtual sizes based
on the CHS algorithm provided by the VHD specs in order to
ensure that the disk geometry (and payload as seen by some
guests which use the CHS value) can fit in the requested disk.

This patch drops this behavior, as it breaks compatibility with
Azure, which requires the MB alignment to be preserved.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>
---
Proposed fix for https://bugs.launchpad.net/qemu/+bug/1490611

 block/vpc.c | 32 ++++++++------------------------
 1 file changed, 8 insertions(+), 24 deletions(-)


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diff --git a/block/vpc.c b/block/vpc.c
index 299d373..77c0a28 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -762,7 +762,6 @@ static int vpc_create(const char *filename, QemuOpts *opts, Error **errp)
     uint8_t buf[1024];
     VHDFooter *footer = (VHDFooter *) buf;
     char *disk_type_param;
-    int i;
     uint16_t cyls = 0;
     uint8_t heads = 0;
     uint8_t secs_per_cyl = 0;
@@ -802,31 +801,16 @@ static int vpc_create(const char *filename, QemuOpts *opts, Error **errp)
         goto out;
     }
 
-    /*
-     * Calculate matching total_size and geometry. Increase the number of
-     * sectors requested until we get enough (or fail). This ensures that
-     * qemu-img convert doesn't truncate images, but rather rounds up.
-     *
-     * If the image size can't be represented by a spec conform CHS geometry,
-     * we set the geometry to 65535 x 16 x 255 (CxHxS) sectors and use
-     * the image size from the VHD footer to calculate total_sectors.
-     */
-    total_sectors = MIN(VHD_MAX_GEOMETRY, total_size / BDRV_SECTOR_SIZE);
-    for (i = 0; total_sectors > (int64_t)cyls * heads * secs_per_cyl; i++) {
-        calculate_geometry(total_sectors + i, &cyls, &heads, &secs_per_cyl);
-    }
-
-    if ((int64_t)cyls * heads * secs_per_cyl == VHD_MAX_GEOMETRY) {
-        total_sectors = total_size / BDRV_SECTOR_SIZE;
+    total_sectors = total_size / BDRV_SECTOR_SIZE;
+    if (total_sectors > VHD_MAX_SECTORS) {
         /* Allow a maximum disk size of approximately 2 TB */
-        if (total_sectors > VHD_MAX_SECTORS) {
-            ret = -EFBIG;
-            goto out;
-        }
-    } else {
-        total_sectors = (int64_t)cyls * heads * secs_per_cyl;
-        total_size = total_sectors * BDRV_SECTOR_SIZE;
+        ret = -EFBIG;
+        goto out;
     }
+    /*
+     * Calculate geometry.
+     */
+    calculate_geometry(total_sectors, &cyls, &heads, &secs_per_cyl);
 
     /* Prepare the Hard Disk Footer */
     memset(buf, 0, 1024);