Einar Lueck <address@hidden> writes:
This patch extends the function hd_geometry_guess. If no geo could
be guessed via guess_disk_lchs, a new function called guess_disk_pchs is
called. The latter utilizes HDIO_GET_GEO ioctl to ask the underlying disk
for geometry.
If this is not successful (e.g. image files) geometry is derived
from the size of the disk (as before).
The new HDIO_GETGEO logic is required for two use cases:
a) Support for geometries of Direct Attached Storage Disks (DASD)
on s390x configured as backing of virtio block devices.
b) Support for FCP attached SCSI disks that do not yet have a
partition table. Without this patch, fdisk -l on the host would
return different results then fdisk -l in the guest.
I'm afraid this could mess up existing, working disks.
Consider a disk where guess_disk_lchs() fails and HDIO_GETGEO succeeds.
The old code arbitrarily picks a "standard" physical geometry then.
Your code picks the one returned by HDIO_GETGEO.
Unless the two geometries happen to lead to a compatible address
translation, any guest that actually uses the translation now sees
different disk contents, with predictably bad results.
Can this happen? If no, why not?