ARC: pt_regs update #3: Remove unused gutter at start of callee_regs

This is trickier than prev two:

* context switching code saves kernel mode callee regs in the format of
  struct callee_regs thus needs adjustment. This also reduces the height
  of topmost kernel stack frame by 1 word.

* Since kernel stack unwinder is sensitive to height of topmost kernel
  stack frame, that needs a word of adjustment too.

ptrace needs a bit of updating since pt_regs now diverges from
user_regs_struct.
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/entry.h

@@ -124,8 +124,6 @@
 	st.a    r25, [sp, -4]
 #endif
 
-	/* move up by 1 word to "create" callee_regs->"stack_place_holder" */
-	sub sp, sp, 4
 .endm
 
 /*--------------------------------------------------------------
@@ -148,10 +146,9 @@
 	st.a    r23, [sp, -4]
 	st.a    r24, [sp, -4]
 #ifdef CONFIG_ARC_CURR_IN_REG
-	sub     sp, sp, 8
+	sub     sp, sp, 4
 #else
 	st.a    r25, [sp, -4]
-	sub     sp, sp, 4
 #endif
 .endm
 
@@ -168,14 +165,11 @@
  *-------------------------------------------------------------*/
 .macro RESTORE_CALLEE_SAVED_KERNEL
 
-
 #ifdef CONFIG_ARC_CURR_IN_REG
-	add     sp, sp, 8  /* skip callee_reg gutter and user r25 placeholder */
+	add     sp, sp, 4  /* skip usual r25 placeholder */
 #else
-	add     sp, sp, 4   /* skip "callee_regs->stack_place_holder" */
 	ld.ab   r25, [sp, 4]
 #endif
-
 	ld.ab   r24, [sp, 4]
 	ld.ab   r23, [sp, 4]
 	ld.ab   r22, [sp, 4]
@@ -203,8 +197,6 @@
  *-------------------------------------------------------------*/
 .macro RESTORE_CALLEE_SAVED_USER
 
-	add     sp, sp, 4   /* skip "callee_regs->stack_place_holder" */
-
 #ifdef CONFIG_ARC_CURR_IN_REG
 	ld.ab   r12, [sp, 4]
 	st      r12, [r25, TASK_THREAD + THREAD_USER_R25]
@@ -230,7 +222,7 @@
  * Super FAST Restore callee saved regs by simply re-adjusting SP
  *-------------------------------------------------------------*/
 .macro DISCARD_CALLEE_SAVED_USER
-	add     sp, sp, 14 * 4
+	add     sp, sp, SZ_CALLEE_REGS
 .endm
 
 /*--------------------------------------------------------------

arch/arc/include/asm/processor.h

@@ -19,6 +19,7 @@
 #ifndef __ASSEMBLY__
 
 #include <asm/arcregs.h>	/* for STATUS_E1_MASK et all */
+#include <asm/ptrace.h>
 
 /* Arch specific stuff which needs to be saved per task.
  * However these items are not so important so as to earn a place in
@@ -75,11 +76,15 @@ unsigned long thread_saved_pc(struct task_struct *t);
 
 /*
  * Where abouts of Task's sp, fp, blink when it was last seen in kernel mode.
- * These can't be derived from pt_regs as that would give correp user-mode val
+ * Look in process.c for details of kernel stack layout
  */
 #define KSTK_ESP(tsk)   (tsk->thread.ksp)
-#define KSTK_BLINK(tsk) (*((unsigned int *)((KSTK_ESP(tsk)) + (13+1+1)*4)))
-#define KSTK_FP(tsk)    (*((unsigned int *)((KSTK_ESP(tsk)) + (13+1)*4)))
+
+#define KSTK_REG(tsk, off)	(*((unsigned int *)(KSTK_ESP(tsk) + \
+					sizeof(struct callee_regs) + off)))
+
+#define KSTK_BLINK(tsk) KSTK_REG(tsk, 4)
+#define KSTK_FP(tsk)    KSTK_REG(tsk, 0)
 
 /*
  * Do necessary setup to start up a newly executed thread.

arch/arc/include/asm/ptrace.h

@@ -59,7 +59,6 @@ struct pt_regs {
 /* Callee saved registers - need to be saved only when you are scheduled out */
 
 struct callee_regs {
-	long res;	/* Again this is not needed */
 	long r25;
 	long r24;
 	long r23;

arch/arc/include/uapi/asm/ptrace.h

@@ -38,8 +38,8 @@ struct user_regs_struct {
 		long r12, r11, r10, r9, r8, r7, r6, r5, r4, r3, r2, r1, r0;
 		long sp;
 	} scratch;
+	long pad2;
 	struct {
-		long pad;
 		long r25, r24, r23, r22, r21, r20;
 		long r19, r18, r17, r16, r15, r14, r13;
 	} callee;

arch/arc/kernel/asm-offsets.c

@@ -60,5 +60,6 @@ int main(void)
 	DEFINE(PT_r6, offsetof(struct pt_regs, r6));
 	DEFINE(PT_r7, offsetof(struct pt_regs, r7));
 
+	DEFINE(SZ_CALLEE_REGS, sizeof(struct callee_regs));
 	return 0;
 }

arch/arc/kernel/ctx_sw.c

@@ -23,10 +23,6 @@ __switch_to(struct task_struct *prev_task, struct task_struct *next_task)
 	unsigned int tmp;
 	unsigned int prev = (unsigned int)prev_task;
 	unsigned int next = (unsigned int)next_task;
-	int num_words_to_skip = 1;
-#ifdef CONFIG_ARC_CURR_IN_REG
-	num_words_to_skip++;
-#endif
 
 	__asm__ __volatile__(
 		/* FP/BLINK save generated by gcc (standard function prologue */
@@ -44,8 +40,9 @@ __switch_to(struct task_struct *prev_task, struct task_struct *next_task)
 		"st.a    r24, [sp, -4]   \n\t"
 #ifndef CONFIG_ARC_CURR_IN_REG
 		"st.a    r25, [sp, -4]   \n\t"
+#else
+		"sub     sp, sp, 4      \n\t"	/* usual r25 placeholder */
 #endif
-		"sub     sp, sp, %4      \n\t"	/* create gutter at top */
 
 		/* set ksp of outgoing task in tsk->thread.ksp */
 		"st.as   sp, [%3, %1]    \n\t"
@@ -76,10 +73,10 @@ __switch_to(struct task_struct *prev_task, struct task_struct *next_task)
 
 		/* start loading it's CALLEE reg file */
 
-		"add    sp, sp, %4     \n\t"	/* skip gutter at top */
-
 #ifndef CONFIG_ARC_CURR_IN_REG
 		"ld.ab   r25, [sp, 4]   \n\t"
+#else
+		"add    sp, sp, 4       \n\t"
 #endif
 		"ld.ab   r24, [sp, 4]   \n\t"
 		"ld.ab   r23, [sp, 4]   \n\t"
@@ -100,8 +97,7 @@ __switch_to(struct task_struct *prev_task, struct task_struct *next_task)
 		/* FP/BLINK restore generated by gcc (standard func epilogue */
 
 		: "=r"(tmp)
-		: "n"((TASK_THREAD + THREAD_KSP) / 4), "r"(next), "r"(prev),
-		  "n"(num_words_to_skip * 4)
+		: "n"((TASK_THREAD + THREAD_KSP) / 4), "r"(next), "r"(prev)
 		: "blink"
 	);
 

arch/arc/kernel/process.c

@@ -55,10 +55,8 @@ asmlinkage void ret_from_fork(void);
  * |     ...        |
  * |    unused      |
  * |                |
- * ------------------  <==== top of Stack (thread.ksp)
- * |   UNUSED 1 word|
  * ------------------
- * |     r25        |
+ * |     r25        |   <==== top of Stack (thread.ksp)
  * ~                ~
  * |    --to--      |   (CALLEE Regs of user mode)
  * |     r13        |

arch/arc/kernel/ptrace.c

@@ -48,6 +48,7 @@ static int genregs_get(struct task_struct *target,
 
 	REG_O_ZERO(pad);
 	REG_O_CHUNK(scratch, callee, ptregs);
+	REG_O_ZERO(pad2);
 	REG_O_CHUNK(callee, efa, cregs);
 	REG_O_CHUNK(efa, stop_pc, &target->thread.fault_address);
 
@@ -96,8 +97,9 @@ static int genregs_set(struct task_struct *target,
 			offsetof(struct user_regs_struct, LOC) + 4);
 
 	REG_IGNORE_ONE(pad);
-	/* TBD: disallow updates to STATUS32, orig_r8 etc*/
-	REG_IN_CHUNK(scratch, callee, ptregs);	/* pt_regs[bta..orig_r8] */
+	/* TBD: disallow updates to STATUS32 etc*/
+	REG_IN_CHUNK(scratch, pad2, ptregs);	/* pt_regs[bta..sp] */
+	REG_IGNORE_ONE(pad2);
 	REG_IN_CHUNK(callee, efa, cregs);	/* callee_regs[r25..r13] */
 	REG_IGNORE_ONE(efa);			/* efa update invalid */
 	REG_IN_ONE(stop_pc, &ptregs->ret);	/* stop_pc: PC update */

arch/arc/kernel/stacktrace.c

@@ -79,7 +79,7 @@ static void seed_unwind_frame_info(struct task_struct *tsk,
 		 * assembly code
 		 */
 		frame_info->regs.r27 = 0;
-		frame_info->regs.r28 += 64;
+		frame_info->regs.r28 += 60;
 		frame_info->call_frame = 0;
 
 	} else {