Hi,
Can there be a guest that will fail the MBR in such a way? Yes.
Look at the following MBR partition table of a Windows XP guest in our production
environment:
Disk size in sectors: 16777216
Binary (only one partition 16 bytes): 80 01 01 00 07 fe ff ff 3f 00 00 00 d5 ea ff 00
Start: (0, 1, 1, 63)
End: (1023, 254, 63, 16771859)
As can be easily seen, any MBR guessing algorithm should guess:
255 heads (since a value of 254 appears), 63 spt (since a value of 63 appears)
Turns out that this image does not work with 255, 63 but actually requires
16 heads, 63 spt
to boot.
So relying on MBR partitions alone is not always enough and sometimes manual intervention
is required.
Ok, given that seabios has no setup any manual configuration needs to be done via qemu.
But why do we need a new interface for that? IDE can pass the geometry
to the guest. virtio-blk has support too (VIRTIO_BLK_F_GEOMETRY).
Likewise scsi (MODE_PAGE_HD_GEOMETRY). So this should be doable without
any qemu changes.