// N.B. since we used a custom stack with makecontext,
	// callbackUpdateSystemStack had to guess the bounds. Its guess assumes
	// a 32KiB stack.
	char *prev_stack_lo = stack2 + STACK_SIZE - (32*1024);

	// New SP is just barely in bounds, but if we don't update the bounds
	// we'll almost certainly overflow. The SP that
	// callbackUpdateSystemStack sees already has some data pushed, so it

	return errno
}

// Set or reset the system stack bounds for a callback on sp.
//
// Must be nosplit because it is called by needm prior to fully initializing
// the M.
//
//go:nosplit
func callbackUpdateSystemStack(mp *m, sp uintptr, signal bool) {
	g0 := mp.g0

	inBound := sp > g0.stack.lo && sp <= g0.stack.hi
	if mp.ncgo > 0 && !inBound {
		// ncgo > 0 indicates that this M was in Go further up the stack

	// would do this if necessary).
	osSetupTLS(mp)

	// Install g (= m->g0) and set the stack bounds
	// to match the current stack.
	setg(mp.g0)
	sp := getcallersp()
	callbackUpdateSystemStack(mp, sp, signal)

	// Should mark we are already in Go now.
	// Otherwise, we may call needm again when we get a signal, before cgocallbackg1,