RISC-V: Add some comments about the shadow and overflow stacks

It took me a while to page all this back in when trying to review the
recent spin_shadow_stack, so I figured I'd just write up some comments.

Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Link: https://lore.kernel.org/r/20221130023515.20217-2-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: palmer-dabbelt

arch/riscv/kernel/traps.c

@@ -200,18 +200,18 @@ void __init trap_init(void)
 }
 
 #ifdef CONFIG_VMAP_STACK
+/*
+ * Extra stack space that allows us to provide panic messages when the kernel
+ * has overflowed its stack.
+ */
 static DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
 		overflow_stack)__aligned(16);
 /*
- * shadow stack, handled_ kernel_ stack_ overflow(in kernel/entry.S) is used
- * to get per-cpu overflow stack(get_overflow_stack).
+ * A temporary stack for use by handle_kernel_stack_overflow.  This is used so
+ * we can call into C code to get the per-hart overflow stack.  Usage of this
+ * stack must be protected by spin_shadow_stack.
  */
 long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE/sizeof(long)] __aligned(16);
-asmlinkage unsigned long get_overflow_stack(void)
-{
-	return (unsigned long)this_cpu_ptr(overflow_stack) +
-		OVERFLOW_STACK_SIZE;
-}
 
 /*
  * A pseudo spinlock to protect the shadow stack from being used by multiple
@@ -222,6 +222,12 @@ asmlinkage unsigned long get_overflow_stack(void)
  */
 unsigned long spin_shadow_stack;
 
+asmlinkage unsigned long get_overflow_stack(void)
+{
+	return (unsigned long)this_cpu_ptr(overflow_stack) +
+		OVERFLOW_STACK_SIZE;
+}
+
 asmlinkage void handle_bad_stack(struct pt_regs *regs)
 {
 	unsigned long tsk_stk = (unsigned long)current->stack;