[Qemu-devel] [PATCH for 1.4] block/vpc: Fix size calculation

[Stefan Weil]

From: Stefan Weil <address@hidden(none)>

The size calculated from the CHS values is not the real image (disk) size,
but usually a smaller value. This is caused by rounding effects.

Only older operating systems use CHS. Such guests won't be able to use
the whole disk. All modern operating systems use the real size.

This patch fixes https://bugs.launchpad.net/qemu/+bug/1105670/.

Signed-off-by: Stefan Weil <address@hidden>
---

This is a rebased extract from my patch series for block/vpc.c.
It's the minimum needed to fix the open bug for QEMU 1.4.

The rest of the series can be discussed and applied after 1.4.

Regards

Stefan W.

PS. Please excuse a previous personal mail which I had sent
with a wrong signature and without addressing qemu-devel.

 block/vpc.c |   14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/block/vpc.c b/block/vpc.c
index 82229ef..b4ff564 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -34,6 +34,8 @@
 
 #define HEADER_SIZE 512
 
+#define VHD_SECTOR_SIZE 512
+
 //#define CACHE
 
 enum vhd_type {
@@ -204,11 +206,13 @@ static int vpc_open(BlockDriverState *bs, int flags)
     /* Write 'checksum' back to footer, or else will leave it with zero. */
     footer->checksum = be32_to_cpu(checksum);
 
-    // The visible size of a image in Virtual PC depends on the geometry
-    // rather than on the size stored in the footer (the size in the footer
-    // is too large usually)
-    bs->total_sectors = (int64_t)
-        be16_to_cpu(footer->cyls) * footer->heads * footer->secs_per_cyl;
+    /* The visible size of a image in Virtual PC depends on the guest:
+     * QEMU and other emulators report the real size (here in sectors).
+     * All modern operating systems use this real size.
+     * Very old operating systems use CHS values to calculate the total size.
+     * This calculated size is usually smaller than the real size.
+     */
+    bs->total_sectors = be64_to_cpu(footer->size) / VHD_SECTOR_SIZE;
 
     /* Allow a maximum disk size of approximately 2 TB */
     if (bs->total_sectors >= 65535LL * 255 * 255) {
-- 
1.7.9.5