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>

arch/arm/include/asm/v7m.h

@@ -49,7 +49,7 @@
  * (0 -> msp; 1 -> psp). Bits [1:0] are fixed to 0b01.
  */
 #define EXC_RET_STACK_MASK			0x00000004
-#define EXC_RET_THREADMODE_PROCESSSTACK		0xfffffffd
+#define EXC_RET_THREADMODE_PROCESSSTACK		(3 << 2)
 
 /* Cache related definitions */
 

arch/arm/kernel/entry-header.S

@@ -127,7 +127,8 @@
          */
 	.macro	v7m_exception_slow_exit ret_r0
 	cpsid	i
-	ldr	lr, =EXC_RET_THREADMODE_PROCESSSTACK
+	ldr	lr, =exc_ret
+	ldr	lr, [lr]
 
 	@ read original r12, sp, lr, pc and xPSR
 	add	r12, sp, #S_IP

arch/arm/kernel/entry-v7m.S

@@ -146,3 +146,7 @@ ENTRY(vector_table)
 	.rept	CONFIG_CPU_V7M_NUM_IRQ
 	.long	__irq_entry		@ External Interrupts
 	.endr
+	.align	2
+	.globl	exc_ret
+exc_ret:
+	.space	4

arch/arm/mm/proc-v7m.S

@@ -139,6 +139,9 @@ __v7m_setup_cont:
 	cpsie	i
 	svc	#0
 1:	cpsid	i
+	ldr	r0, =exc_ret
+	orr	lr, lr, #EXC_RET_THREADMODE_PROCESSSTACK
+	str	lr, [r0]
 	ldmia	sp, {r0-r3, r12}
 	str	r5, [r12, #11 * 4]	@ restore the original SVC vector entry
 	mov	lr, r6			@ restore LR