ARC: pt_regs update #4: r25 saved/restored unconditionally

(This is a VERY IMP change for low level interrupt/exception handling)

-----------------------------------------------------------------------
WHAT
-----------------------------------------------------------------------
* User 25 now saved in pt_regs->user_r25 (vs. tsk->thread_info.user_r25)

* This allows Low level interrupt code to unconditionally save r25
  (vs. the prev version which would only do it for U->K transition).
  Ofcourse for nested interrupts, only the pt_regs->user_r25 of
  bottom-most frame is useful.

* simplifies the interrupt prologue/epilogue

* Needed for ARCv2 ISA code and done here to keep design similar with
  ARCompact event handling

-----------------------------------------------------------------------
WHY
-------------------------------------------------------------------------
With CONFIG_ARC_CURR_IN_REG, r25 is used to cache "current" task pointer
in kernel mode. So when entering kernel mode from User Mode
- user r25 is specially safe-kept (it being a callee reg is NOT part of
  pt_regs which are saved by default on each interrupt/trap/exception)
- r25 loaded with current task pointer.

Further, if interrupt was taken in kernel mode, this is skipped since we
know that r25 already has valid "current" pointer.

With 2 level of interrupts in ARCompact ISA, detecting this is difficult
but still possible, since we could be in kernel mode but r25 not already saved
(in fact the stack itself might not have been switched).

A. User mode
B. L1 IRQ taken
C. L2 IRQ taken (while on 1st line of L1 ISR)

So in #C, although in kernel mode, r25 not saved (infact SP not
switched at all)

Given that ARcompact has manual stack switching, we could use a bit of
trickey - The low level code would make sure that SP is only set to kernel
mode value at the very end (after saving r25). So a non kernel mode SP,
even if in kernel mode, meant r25 was NOT saved.

The same paradigm won't work in ARCv2 ISA since SP is auto-switched so
it's setting can't be delayed/constrained.
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/entry.h

@@ -102,6 +102,10 @@
 	POP	r2
 	POP	r1
 	POP	r0
+
+#ifdef CONFIG_ARC_CURR_IN_REG
+	ld	r25, [sp, 12]
+#endif
 .endm
 
 /*--------------------------------------------------------------
@@ -138,6 +142,7 @@
 	POP	r13
 .endm
 
+#define OFF_USER_R25_FROM_R24	(SZ_CALLEE_REGS + SZ_PT_REGS - 8)/4
 
 /*--------------------------------------------------------------
  * Collect User Mode callee regs as struct callee_regs - needed by
@@ -155,7 +160,7 @@
 
 #ifdef CONFIG_ARC_CURR_IN_REG
 	; Retrieve orig r25 and save it on stack
-	ld      r12, [r25, TASK_THREAD + THREAD_USER_R25]
+	ld.as   r12, [sp, OFF_USER_R25_FROM_R24]
 	st.a    r12, [sp, -4]
 #else
 	PUSH	r25
@@ -204,7 +209,7 @@
 
 #ifdef CONFIG_ARC_CURR_IN_REG
 	ld.ab   r12, [sp, 4]
-	st      r12, [r25, TASK_THREAD + THREAD_USER_R25]
+	st.as   r12, [sp, OFF_USER_R25_FROM_R24]
 #else
 	POP	r25
 #endif
@@ -218,13 +223,6 @@
 	add     sp, sp, SZ_CALLEE_REGS
 .endm
 
-/*--------------------------------------------------------------
- * Restore User mode r25 saved in task_struct->thread.user_r25
- *-------------------------------------------------------------*/
-.macro RESTORE_USER_R25
-	ld  r25, [r25, TASK_THREAD + THREAD_USER_R25]
-.endm
-
 /*-------------------------------------------------------------
  * given a tsk struct, get to the base of it's kernel mode stack
  * tsk->thread_info is really a PAGE, whose bottom hoists stack
@@ -297,22 +295,21 @@
 
 	GET_CURR_TASK_ON_CPU   r9
 
-#ifdef CONFIG_ARC_CURR_IN_REG
-
-	/* If current task pointer cached in r25, time to
-	 *  -safekeep USER r25 in task->thread_struct->user_r25
-	 *  -load r25 with current task ptr
-	 */
-	st.as	r25, [r9, (TASK_THREAD + THREAD_USER_R25)/4]
-	mov	r25, r9
-#endif
-
 	/* With current tsk in r9, get it's kernel mode stack base */
 	GET_TSK_STACK_BASE  r9, r9
 
 66:
+#ifdef CONFIG_ARC_CURR_IN_REG
+	/*
+	 * Treat r25 as scratch reg, save it on stack first
+	 * Load it with current task pointer
+	 */
+	st	r25, [r9, -4]
+	GET_CURR_TASK_ON_CPU   r25
+#endif
+
 	/* Save Pre Intr/Exception User SP on kernel stack */
-	st.a    sp, [r9, -12]	; Make room for orig_r0 and orig_r8
+	st.a    sp, [r9, -16]	; Make room for orig_r0, orig_r8, user_r25
 
 	/* CAUTION:
 	 * SP should be set at the very end when we are done with everything
@@ -466,7 +463,7 @@
 	RESTORE_R12_TO_R0
 
 	ld  sp, [sp] /* restore original sp */
-	/* orig_r0 and orig_r8 skipped automatically */
+	/* orig_r0, orig_r8, user_r25 skipped automatically */
 .endm
 
 
@@ -549,7 +546,7 @@
 	RESTORE_R12_TO_R0
 
 	ld  sp, [sp] /* restore original sp */
-	/* orig_r0 and orig_r8 skipped automatically */
+	/* orig_r0, orig_r8, user_r25 skipped automatically */
 .endm
 
 .macro RESTORE_ALL_INT2
@@ -568,7 +565,7 @@
 	RESTORE_R12_TO_R0
 
 	ld  sp, [sp] /* restore original sp */
-	/* orig_r0 and orig_r8 skipped automatically */
+	/* orig_r0, orig_r8, user_r25 skipped automatically */
 .endm
 
 

arch/arc/include/asm/processor.h

@@ -30,9 +30,6 @@ struct thread_struct {
 	unsigned long callee_reg;	/* pointer to callee regs */
 	unsigned long fault_address;	/* dbls as brkpt holder as well */
 	unsigned long cause_code;	/* Exception Cause Code (ECR) */
-#ifdef CONFIG_ARC_CURR_IN_REG
-	unsigned long user_r25;
-#endif
 #ifdef CONFIG_ARC_FPU_SAVE_RESTORE
 	struct arc_fpu fpu;
 #endif

arch/arc/include/asm/ptrace.h

@@ -54,6 +54,8 @@ struct pt_regs {
 #endif
 		long orig_r8_word;
 	};
+
+	long user_r25;
 };
 
 /* Callee saved registers - need to be saved only when you are scheduled out */

arch/arc/kernel/asm-offsets.c

@@ -24,9 +24,6 @@ int main(void)
 
 	DEFINE(THREAD_KSP, offsetof(struct thread_struct, ksp));
 	DEFINE(THREAD_CALLEE_REG, offsetof(struct thread_struct, callee_reg));
-#ifdef CONFIG_ARC_CURR_IN_REG
-	DEFINE(THREAD_USER_R25, offsetof(struct thread_struct, user_r25));
-#endif
 	DEFINE(THREAD_FAULT_ADDR,
 	       offsetof(struct thread_struct, fault_address));
 
@@ -61,5 +58,6 @@ int main(void)
 	DEFINE(PT_r7, offsetof(struct pt_regs, r7));
 
 	DEFINE(SZ_CALLEE_REGS, sizeof(struct callee_regs));
+	DEFINE(SZ_PT_REGS, sizeof(struct pt_regs));
 	return 0;
 }

arch/arc/kernel/entry.S

@@ -680,17 +680,6 @@ restore_regs :
 	; XXX can this be optimised out
 	IRQ_DISABLE_SAVE    r9, r10	;@r10 has prisitine (pre-disable) copy
 
-#ifdef CONFIG_ARC_CURR_IN_REG
-	; Restore User R25
-	; Earlier this used to be only for returning to user mode
-	; However with 2 levels of IRQ this can also happen even if
-	; in kernel mode
-	ld r9, [sp, PT_sp]
-	brhs r9, VMALLOC_START, 8f
-	RESTORE_USER_R25
-8:
-#endif
-
 	; Restore REG File. In case multiple Events outstanding,
 	; use the same priorty as rtie: EXCPN, L2 IRQ, L1 IRQ, None
 	; Note that we use realtime STATUS32 (not pt_regs->status32) to

arch/arc/kernel/process.c

@@ -77,6 +77,7 @@ asmlinkage void ret_from_fork(void);
  * |      SP        |
  * |    orig_r0     |
  * |    orig_r8     |
+ * |    user_r25    |
  * ------------------  <===== END of PAGE
  */
 int copy_thread(unsigned long clone_flags,