Hi !
Let's first consider ARMv8 aarch64 VMSA and stage 1 Table descriptors. I can read in the ARM ARM in 'Next-level attributes in stage 1 VMSAv8-64 Table descriptors' that bits [58:52] are Ignored. More precisely we can read:
The definition of IGNORED means the architecture guarantees that the PE makes no use of the field
However in the section about IGNORED fields, we can read that they are ignored, but it is not stated that they are reserved for software use.
In ARMv7a / LPAE, they are only written as IGNORED.
My question is the following: can I use this bits as part of my software MMU management ? Since they are IGNORED but not reserved for sotfware, I'm not sure.
Best regards,
V
In D4.5.3 under 'IGNORED fields' it says in the stage 1 and stage 2 block and page descriptors, bits[58:55] are reserved for software use.
Yes, I saw that, it is why I explicitly ask the question for table descriptors.
For the definition of IGNORED, I can't see a difference between "IGNORED" and "reserved for software use", that's why I need help :)
When it says 'IGNORED' in the architectural description it is not guaranteeing that ARMv8.4 or some other extension not yet defined or made public will not use the bits. If software uses bits and they are not reserved for software use then there may be some problem down the line with future chips. If they have been explicitly reserved for software use though ARM would have to have a very good reason indeed for trying to claw them back and would deserve any flak they got.
Ok, I understand. Thank you for this detailed answer !