ARM: 8830/1: NOMMU: Toggle only bits in EXC_RETURN we are really care of
ARMv8M introduces support for Security extension to M class, among
other things it affects exception handling, especially, encoding of
EXC_RETURN.
The new bits have been added:
Bit [6] Secure or Non-secure stack
Bit [5] Default callee register stacking
Bit [0] Exception Secure
which conflicts with hard-coded value of EXC_RETURN:
In fact, we only care of few bits:
Bit [3] Mode (0 - Handler, 1 - Thread)
Bit [2] Stack pointer selection (0 - Main, 1 - Process)
We can toggle only those bits and left other bits as they were on
exception entry.
It is basically, what patch does - saves EXC_RETURN when we do
transition form Thread to Handler mode (it is first svc), so later
saved value is used instead of EXC_RET_THREADMODE_PROCESSSTACK.
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com> Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>