geometry: Read LCHS from fw_cfg

Read boot device information from fw_cfg.

Boot device information will contain logical geometry (LCHS) values,
but it is implemented in a manner which allows extension.

By receiving LCHS values directly from QEMU through fw_cfg we will be
able to support logical geometries which can not be inferred by SeaBIOS
(For instance: A 8GB virtio-blk hard drive which was originally created
as an IDE and must report LCHS of */32/63 for its operating system to
function will always break under SeaBIOS since a LARGE/LBA translation
will be used, causing the number of reported logical heads to be > 32.)

The only LCHS paravirtual interface available at the moment is for IDE
disks (rtc_read() in get_translation()) and it's limited to a maximum
of 4 disks (this code existed in SeaBIOS's translation function before
SCSI and VirtIO were even introduced).
This is why we create a new interface which allows passing LCHS
information per hdd. As mentioned, this interface may be easily extended
to support more information per hdd.

Boot device information is serialized in the following way:
    * struct_size (u32)
    * device path (sz string)
    * device information (struct_size)
    * device path (sz string)
    * device information (struct_size)

Device path is a null terminated string in the "Open Firmware" device
path format, the same path as used in bootorder.

Reviewed-by: Karl Heubaum <>
Reviewed-by: Arbel Moshe <>
Signed-off-by: Sam Eiderman <>
Message-Id: <>
Signed-off-by: Gerd Hoffmann <>
1 file changed
tree: 1c6f05c10e4c2b123321144a6199ba83d250eaa9
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