Not a huge deal, but at a minimum it seems a generic construct thatSurely there's an existing macro for this pattern? Would
PUSH_AND_CLEAR_REGS + POP_REGS be suitable? Besides code sharing it
would eliminate clearing of %r8.
There used to be SAVE_ALL/SAVE_REGS, but they have been all removed in
some past refactorings.
deserves to be declared centrally rather than tdx-guest-port-io local.