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ARM: 8830/1: NOMMU: Toggle only bits in EXC_RETURN we are really care of

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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>
This commit is contained in:
Vladimir Murzin 2019-01-25 15:18:37 +01:00 • committed by Russell King
commit 72cd4064fc
4 changed files with 10 additions and 2 deletions
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