tilegx: change how we find the kernel stack

Previously, we used a special-purpose register (SPR_SYSTEM_SAVE_K_0)
to hold the CPU number and the top of the current kernel stack
by using the low bits to hold the CPU number, and using the high
bits to hold the address of the page just above where we'd want
the kernel stack to be.  That way we could initialize a new SP
when first entering the kernel by just masking the SPR value and
subtracting a couple of words.

However, it's actually more useful to be able to place an arbitrary
kernel-top value in the SPR.  This allows us to create a new stack
context (e.g. for virtualization) with an arbitrary top-of-stack VA.
To make this work, we now store the CPU number in the high bits,
above the highest legal VA bit (42 bits in the current tilegx
microarchitecture).  The full 42 bits are thus available to store the
top of stack value.  Getting the current cpu (a relatively common
operation) is still fast; it's now a shift rather than a mask.

We make this change only for tilegx, since tilepro has too few SPR
bits to do this, and we don't need this support on tilepro anyway.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

1 files changed, 33 insertions, 14 deletions

diff --git a/arch/tile/include/asm/processor.h b/arch/tile/include/asm/processor.h
index 461322b473b5..230b830e94d4 100644
--- a/arch/tile/include/asm/processor.h
+++ b/arch/tile/include/asm/processor.h
@@ -148,9 +148,10 @@ struct thread_struct {
 
 /*
  * Start with "sp" this many bytes below the top of the kernel stack.
- * This preserves the invariant that a called function may write to *sp.
+ * This allows us to be cache-aware when handling the initial save
+ * of the pt_regs value to the stack.
  */
-#define STACK_TOP_DELTA 8
+#define STACK_TOP_DELTA 64
 
 /*
  * When entering the kernel via a fault, start with the top of the
@@ -234,15 +235,15 @@ extern int do_work_pending(struct pt_regs *regs, u32 flags);
 unsigned long get_wchan(struct task_struct *p);
 
 /* Return initial ksp value for given task. */
-#define task_ksp0(task) ((unsigned long)(task)->stack + THREAD_SIZE)
+#define task_ksp0(task) \
+	((unsigned long)(task)->stack + THREAD_SIZE - STACK_TOP_DELTA)
 
 /* Return some info about the user process TASK. */
-#define KSTK_TOP(task)	(task_ksp0(task) - STACK_TOP_DELTA)
 #define task_pt_regs(task) \
-  ((struct pt_regs *)(task_ksp0(task) - KSTK_PTREGS_GAP) - 1)
+	((struct pt_regs *)(task_ksp0(task) - KSTK_PTREGS_GAP) - 1)
 #define current_pt_regs()                                   \
-  ((struct pt_regs *)((stack_pointer | (THREAD_SIZE - 1)) - \
-                      (KSTK_PTREGS_GAP - 1)) - 1)
+	((struct pt_regs *)((stack_pointer | (THREAD_SIZE - 1)) - \
+			    STACK_TOP_DELTA - (KSTK_PTREGS_GAP - 1)) - 1)
 #define task_sp(task)	(task_pt_regs(task)->sp)
 #define task_pc(task)	(task_pt_regs(task)->pc)
 /* Aliases for pc and sp (used in fs/proc/array.c) */
@@ -355,20 +356,38 @@ extern int kdata_huge;
 #define KERNEL_PL CONFIG_KERNEL_PL
 
 /* SYSTEM_SAVE_K_0 holds the current cpu number ORed with ksp0. */
-#define CPU_LOG_MASK_VALUE 12
-#define CPU_MASK_VALUE ((1 << CPU_LOG_MASK_VALUE) - 1)
-#if CONFIG_NR_CPUS > CPU_MASK_VALUE
-# error Too many cpus!
+#ifdef __tilegx__
+#define CPU_SHIFT 48
+#if CHIP_VA_WIDTH() > CPU_SHIFT
+# error Too many VA bits!
 #endif
+#define MAX_CPU_ID ((1 << (64 - CPU_SHIFT)) - 1)
 #define raw_smp_processor_id() \
-	((int)__insn_mfspr(SPR_SYSTEM_SAVE_K_0) & CPU_MASK_VALUE)
+	((int)(__insn_mfspr(SPR_SYSTEM_SAVE_K_0) >> CPU_SHIFT))
 #define get_current_ksp0() \
-	(__insn_mfspr(SPR_SYSTEM_SAVE_K_0) & ~CPU_MASK_VALUE)
+	((unsigned long)(((long)__insn_mfspr(SPR_SYSTEM_SAVE_K_0) << \
+			  (64 - CPU_SHIFT)) >> (64 - CPU_SHIFT)))
+#define next_current_ksp0(task) ({ \
+	unsigned long __ksp0 = task_ksp0(task) & ((1UL << CPU_SHIFT) - 1); \
+	unsigned long __cpu = (long)raw_smp_processor_id() << CPU_SHIFT; \
+	__ksp0 | __cpu; \
+})
+#else
+#define LOG2_NR_CPU_IDS 6
+#define MAX_CPU_ID ((1 << LOG2_NR_CPU_IDS) - 1)
+#define raw_smp_processor_id() \
+	((int)__insn_mfspr(SPR_SYSTEM_SAVE_K_0) & MAX_CPU_ID)
+#define get_current_ksp0() \
+	(__insn_mfspr(SPR_SYSTEM_SAVE_K_0) & ~MAX_CPU_ID)
 #define next_current_ksp0(task) ({ \
 	unsigned long __ksp0 = task_ksp0(task); \
 	int __cpu = raw_smp_processor_id(); \
-	BUG_ON(__ksp0 & CPU_MASK_VALUE); \
+	BUG_ON(__ksp0 & MAX_CPU_ID); \
 	__ksp0 | __cpu; \
 })
+#endif
+#if CONFIG_NR_CPUS > (MAX_CPU_ID + 1)
+# error Too many cpus!
+#endif
 
 #endif /* _ASM_TILE_PROCESSOR_H */