Previous RFC here
https://lists.gnu.org/archive/html/qemu-ppc/2023-05/msg00453.html
This series drops patch 1 from the previous, which is more of
a standalone bugfix.
Also accounted for Cedric's comments, except a nicer way to
set cpu_index vs PIR/TIR SPRs, which is not quite trivial.
This limits support for SMT to POWER8 and newer. It is also
incompatible with nested-HV so that is checked for too.
Iterating CPUs to find siblings for now I kept because similar
loops exist in a few places, and it is not conceptually
difficult for SMT, just fiddly code to improve. For now it
should not be much performane concern.
I removed hypervisor msgsnd support from patch 3, which is not
required for spapr and added significantly to the patch.
For now nobody has objected to the way shared SPR access is
handled (serialised with TCG atomics support) so we'll keep
going with it.