Re: [Qemu-devel] [QEMU] [PATCH v2 0/8] Add Qemu to SeaBIOS LCHS interface

[Gerd Hoffmann]

  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.

cheers,
  Gerd