Hi Igor, On 23.12.21 15:49, Igor Bagnucki wrote: > > 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.
how likely is it that an OS runs in big-endian? IIRC from past discussions it was usually expected that the OS runs in little- endian. If there is such a preference, I'd simply specify the cbtable endianness to be little-endian. After all, the cbtable is an interface for the OS so we should make things easy on that end, IMHO. Also, if we could assume little-endian OS's, we could also switch early (e.g. bootblock) to that. Or is there any other technical difficulty why we don't do that already? > 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. 5. Specify a single endianness for cbtable. I would prefer little-endian as that would keep compatibility to older (maybe not too old) user-space binaries. Even if we have code that runs in big-endian, that would still keep the conversion simple. We could just always convert host-to-LE and vice versa, no need for any `if`. Nico > > [1] https://github.com/Dasharo/coreboot/pull/62 > _______________________________________________ coreboot mailing list -- coreboot@coreboot.org To unsubscribe send an email to coreboot-le...@coreboot.org
