Concerns with hypercall table changes

Description:

Potential concerns with the series xen: drop hypercall function tables [1]:

• Something from the guest is shifted more than 64 bits (UB)
• Highly likely perf issue on ARM; probably on x86 going forward
• Incompatible with future intended ABI changes; specifically, one way or another, we're going to have to use indices higher than 64.
• kexec_op ABI broken (regression vs 4.16); changes a bunch of unsigned logs to unsigned ints, truncating the input value to 32 bits; some values which were previously rejected are accepted and aliased

Jan: Agree we may want to revert it at some point. But some of the grant table hypercall changes rely on some of Juergen's series (replacing do_grnat_table_op() with a shim-special thing)

[1] https://patchew.org/Xen/20220324140139.5899-1-jgross@suse.com/

Outcomes necessary to resolve

• Shift UB issue addressed (either shown not problematic or fixed)
• Perf issue addressed (either data, or a convincing argument either way)
• kexec_op ABI issue addressed (either shown not problematic or fixed)
• Future ABI issue addressed (either shown not problematic or fixed)

Next step(s)

• (WHO?) Describe UB shift issue sufficiently to agree that it's problematic
• Figure out what the next steps are for the other two issues