QEMU guest system starts off /dev/sdc but won’t start from disk image file

Question

I have a copy of Windows XP installed on a hard drive which I would like to run with QEMU.

When I start it directly from the HDD it runs fine:

qemu -m 256 -hda /dev/sdc
Starting Windows...etc...etc...

Although the HDD capacity is 320GB, my system actually lives in a 4GB partition at the beginning of the disk:

/sbin/fdisk -l /dev/sdc

Disk /dev/sdc: 320 GB, 320070320640 bytes
255 heads, 63 sectors/track, 38913 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1   *           1         510     4096543    b  FAT32
Warning: Partition 1 does not end on cylinder boundary.

So I figured I could only copy the partition table and the first partition (8192000 sectors) to a raw image for use with QEMU:

dd if=/dev/sdc of=winxp.img count=8192000

Of course, since winxp.img is a copy of /dev/sdc, it has exactly the same partition layout:

/sbin/fdisk -l winxp.img

Disk /mnt/windata/winxp.raw: 4 GB, 4186667520 bytes
255 heads, 63 sectors/track, 509 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

                 Device Boot      Start         End      Blocks   Id  System
/mnt/windata/winxp.raw1   *           1         510     4096543    b  FAT32
Warning: Partition 1 does not end on cylinder boundary.

However, when I try to use the raw image with QEMU, the guest system won’t start:

qemu -m 256 -hda winxp.img
Booting from hard disk...
NTLDR is missing

What settings could I change to make my image file compatible with QEMU? Should I change something in the partition table, or in the partition itself, to make it work? Is there a QEMU option that could help?

Or is it perhaps possible to use a copy of /dev/sdc1 with QEMU instead of a copy of /dev/sdc?

Answer 1

It appears that SeaBIOS (used by QEMU when emulating PC targets) has no manual configuration and always decides drive geometry automatically. Since my partial image is quite small, its CHS geometry is detected as 8129 * 16 * 63 rather than 510 * 255 * 63. This is done for compatibility with old drives which didn't understand LBA and thus were limited to 16 heads.

Because of the mismatch between the drive geometry in the MBR and BIOS, the bootloader fails to boot windows properly.

Theoretically, images of more than 8GB should not be affected (I know my 320GB image worked), since drives with more than 8GB have to support LBA. Unfortunately, I don't have that image anymore to test this hypothesis.

Answer 2

if you can 'mount' your image file to a windows machine (XP preferrably), you can use Microsoft tools against it to see what the problem might be. the partition has to be 'active', it has to be an MBR (not GPT) formatted disk, and it has to have the correct boot sector.

tools like diskpart and bootsect /nt52 /mbr have been useful for me in the past. I'm not familiar with booting windows from a Linux partition.

Answer 3

Just a guess: you are not getting the MBR as I think you figured out already. The whole disk is too big. so....

1) grab a USB drive with enough space
get a working disk image;
convert to qcow2;
shrink it.

2) use gparted or parted to shrink the original.
e2fsck and tune2fs *may* be helpful, also resize2fs.