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

[Sam Eiderman]

> On 14 Jun 2019, at 7:43, Gerd Hoffmann <address@hidden> wrote:
> 
>  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.

This was indeed considered (all 3 methods) but it has the following issues:

Physical geometries of devices must now also be logical geometries with 
translation=none.
When the OS will query these devices - It will now see different physical 
geometries, adapted to be logical geometries.
I’m not sure even how to implement this without breaking existing compatibility 
- since we don’t want to affect logical geometries of currently used guests.
MODE_PAGE_HD_GEOMETRY does not contain the spts, only cylinders (as 3 byte 
number) and heads (as 1 byte number) and computes the spts using: 
number_of_total_sectors / (heads * cylinders), this means that qemu now must 
report number_of_total_sectors as heads * cylinders * spt for SeaBIOS to 
correctly receive the number of spts - this is not optimal since 
number_of_total_sectors can not reflect the real amount of sectors in the disk 
which should be reported from CDB_CMD_READ_CAPACITY.
Moving a scsi-hd/virtio-blk with 255 physical heads to ide-hd, we will still 
need to report 255 heads - this is possible since a whole byte can be used in 
the “ide identify” command, but goes against the spec of a maximum of 16 heads 
for IDE.

Overall this approach is much more complicated.

Sam

> 
> cheers,
>  Gerd
>