1. Regarding save_x0_x1, it is 0 only for guest_sync_slowpath, which is called
by guest_sync.
So as we are dealing only with compat=1, save_x0_x1 cannot be 0.
The conclusion is that we do not need to worry about it.
Oh, good point. I guess you may want to add a build time check to
avoid silently introducing a user of the macro violating that
assumption.
