Dear coreboot community,
as a part of coreboot port for Talos II POWER9 architecture, we are
porting the cbmem tool. [1] This platform can switch between the
big-endian and the little-endian. coreboot operates in big-endian but in
theory it could also operate in little-endian, and the OS can also
operate in both - little-endian and big-endian. This creates a situation
where cbmem tries to read cbtable in different endianness than the
cbtable was created. Therefore, cbmem needs to convert the endianness,
and it needs to differentiate if the conversion is required or not.
There are several ways we could differentiate if it is required to
convert endianness or not. What would be the best approach to this problem?
1. Save endianness information in the cbtable header.
For example, the signature on the big-endian coreboot could be “OIBL”
instead “LBIO”. This solution would be very portable as the other tools
looking for the cbtable but not expecting the big-endian format would
simply not find the table instead of trying to read it and crashing.
However, this change would need to be introduced in multiple places in
coreboot as the signature is hardcoded in multiple where it was needed.
2. Add preprocessor checks to enable endianness conversion only on
defined platforms.
It would be easy to implement, but if the BIOS and the OS can have
different endianness, then it could break for cbtable written in
little-endian mode.
3. Recognize the correct endianness by checking some property of cbtable
header. For example, header size property value could be checked if it
reads as 0x18 or 0x18000000, but this solution requires very little
changes but feels hacky and susceptible to changes in cbtable header format.
4. Add command-line option to cbmem so the user can choose what
endianness should be used. I’m not sure about this solution, because it
pushes the responsibility to recognize endianness to the user.
[1] https://github.com/Dasharo/coreboot/pull/62
--
Igor Bagnucki
Embedded Firmware Developer
https://3mdeb.com | @3mdeb_com
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