ARC: [SMP] Fix build failures for large NR_CPUS

ST.as only takes S9 (255) for offset. This was going out of range when
accessing a task_struct field with 4k NR_CPUS (due to 128b of coumaks
itself in there).

Workaround by using an intermediate register to do the address scaling.

There is some duplication of fix for ctx_sw.c and ctx_sw_asm.S however
given that C version will go away soon I'm not bothering to factor out
the common code.

Reported-by: Noam Camus <noamc@ezchip.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

2 files changed, 21 insertions, 3 deletions

diff --git a/arch/arc/kernel/ctx_sw.c b/arch/arc/kernel/ctx_sw.c
index 34410eb1a308..c14a5bea0c76 100644
--- a/arch/arc/kernel/ctx_sw.c
+++ b/arch/arc/kernel/ctx_sw.c
@@ -17,6 +17,8 @@
 #include <asm/asm-offsets.h>
 #include <linux/sched.h>
 
+#define KSP_WORD_OFF    ((TASK_THREAD + THREAD_KSP) / 4)
+
 struct task_struct *__sched
 __switch_to(struct task_struct *prev_task, struct task_struct *next_task)
 {
@@ -45,7 +47,16 @@ __switch_to(struct task_struct *prev_task, struct task_struct *next_task)
 #endif
 
                 /* set ksp of outgoing task in tsk->thread.ksp */
+#if KSP_WORD_OFF <= 255
                 "st.as   sp, [%3, %1]    \n\t"
+#else
+                /*
+                 * Workaround for NR_CPUS=4k
+                 * %1 is bigger than 255 (S9 offset for st.as)
+                 */
+                "add2    r24, %3, %1     \n\t"
+                "st      sp, [r24]       \n\t"
+#endif
 
                 "sync   \n\t"
 
@@ -97,7 +108,7 @@ __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"(KSP_WORD_OFF), "r"(next), "r"(prev)
                 : "blink"
         );
 
diff --git a/arch/arc/kernel/ctx_sw_asm.S b/arch/arc/kernel/ctx_sw_asm.S
index d8972345e4c2..65690e7fcc8c 100644
--- a/arch/arc/kernel/ctx_sw_asm.S
+++ b/arch/arc/kernel/ctx_sw_asm.S
@@ -14,6 +14,8 @@
 #include <asm/asm-offsets.h>
 #include <asm/linkage.h>
 
+#define KSP_WORD_OFF    ((TASK_THREAD + THREAD_KSP) / 4)
+
 ;################### Low Level Context Switch ##########################
 
         .section .sched.text,"ax",@progbits
@@ -28,8 +30,13 @@ __switch_to:
         SAVE_CALLEE_SAVED_KERNEL
 
         /* Save the now KSP in task->thread.ksp */
-        st.as  sp, [r0, (TASK_THREAD + THREAD_KSP)/4]
-
+#if KSP_WORD_OFF  <= 255
+        st.as  sp, [r0, KSP_WORD_OFF]
+#else
+        /* Workaround for NR_CPUS=4k as ST.as can only take s9 offset */
+        add2    r24, r0, KSP_WORD_OFF
+        st      sp, [r24]
+#endif
         /*
         * Return last task in r0 (return reg)
         * On ARC, Return reg = First Arg reg = r0.