https://github.com/smithp35 commented:

My apologies for the delay, managed to catch me at a busy time in the day job. 
Will be out of office till next week so may be a bit slow to reply.

I appreciate that you've included the documentation in the patch, I've put some 
questions there that probably don't affect this patch, but may affect later 
ones. I can't spot anything obviously wrong in this patch.

The one important decision in this patch is the choice of a subtarget for LFI. 
I guess the most obvious alternatives would be a LFI envionment (like 
pauthtest), or an ABI (like aapcs-soft). LFI is a bit strange in that it is 
kind of a mixture of all three. The instructions are restricted to what the 
verifier can accept, the syscalls are limited to what the sandbox accepts, and 
there is a different procedure call standard. Given that objects compiled for 
LFI won't be compatible with non-LFI I think a sub-target is probably the best 
choice, or at least I can't think of a better one.

Your suggestions of how to split the patch sound reasonable to me. Without 
spending a lot of effort looking at the full patch, it is diffcult to know if 
there is a significantly better way. I guess we'll let you know if it needs 
splitting again.

I expect to have more comments on later patches. Particularly if there's 
anything we need to document in the Arm ABI. For example identifying 
relocatable objects as LFI (to diagnose clashes with non-LFI at link-time) and 
identifying executables as requiring LFI. 

https://github.com/llvm/llvm-project/pull/167061
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