Re: [Qemu-devel] [PATCH V2] hd-geometry.c: Integrate HDIO_GETGEO in guessing

[Einar Lueck <elelueck@linux.vnet.ibm.com>]

On 01/14/2013 02:23 PM, Markus Armbruster wrote:
> Einar Lueck <elelueck@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?
>

You are right. There may be such cases. Thus, I see two options:
a) making this code portion platform specific (s390x)
b) introduction of a a command line option to force reconsideration of 
HDIO_GETGEO
My impression is, that the value of b) is limited to s390x and therefore 
a) is the way to go.

Regards,
Einar.