Einar Lueck <elelu...@linux.vnet.ibm.com> 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?
