On Mon, Oct 11, 2021 at 03:19:37PM +0100, Szabolcs Nagy wrote:

if fork resets tpidr2 then returning to a stack frame
that set tpidr2 could cause clobbered za state.
(userspace can fix this up, but if we want this case
to work then it's better to fix it on the kernel side)

OK, that makes sense.  I've changed the code and the kselftest so that
TPIDR2 is preserved on thread creation.