arm64: entry: add a call_on_irq_stack helper

When handling IRQ/FIQ exceptions the entry assembly may transition from
a task's stack to a CPU's IRQ stack (and IRQ shadow call stack).

In subsequent patches we want to migrate the IRQ/FIQ triage logic to C,
and as we want to perform some actions on the task stack (e.g. EL1
preemption), we need to switch stacks within the C handler. So that we
can do so, this patch adds a helper to call a function on a CPU's IRQ
stack (and shadow stack as appropriate).

Subsequent patches will make use of the new helper function.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210607094624.34689-7-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/exception.h

@@ -40,6 +40,8 @@ asmlinkage void el0_error_compat_handler(struct pt_regs *regs);
 
 asmlinkage void noinstr enter_el1_irq_or_nmi(struct pt_regs *regs);
 asmlinkage void noinstr exit_el1_irq_or_nmi(struct pt_regs *regs);
+asmlinkage void call_on_irq_stack(struct pt_regs *regs,
+				  void (*func)(struct pt_regs *));
 asmlinkage void enter_from_user_mode(void);
 asmlinkage void exit_to_user_mode(void);
 void arm64_enter_nmi(struct pt_regs *regs);

arch/arm64/kernel/entry.S

@@ -982,6 +982,42 @@ SYM_CODE_START(ret_from_fork)
 SYM_CODE_END(ret_from_fork)
 NOKPROBE(ret_from_fork)
 
+/*
+ * void call_on_irq_stack(struct pt_regs *regs,
+ * 		          void (*func)(struct pt_regs *));
+ *
+ * Calls func(regs) using this CPU's irq stack and shadow irq stack.
+ */
+SYM_FUNC_START(call_on_irq_stack)
+#ifdef CONFIG_SHADOW_CALL_STACK
+	stp	scs_sp, xzr, [sp, #-16]!
+	ldr_this_cpu scs_sp, irq_shadow_call_stack_ptr, x17
+#endif
+	/* Create a frame record to save our LR and SP (implicit in FP) */
+	stp	x29, x30, [sp, #-16]!
+	mov	x29, sp
+
+	ldr_this_cpu x16, irq_stack_ptr, x17
+	mov	x15, #IRQ_STACK_SIZE
+	add	x16, x16, x15
+
+	/* Move to the new stack and call the function there */
+	mov	sp, x16
+	blr	x1
+
+	/*
+	 * Restore the SP from the FP, and restore the FP and LR from the frame
+	 * record.
+	 */
+	mov	sp, x29
+	ldp	x29, x30, [sp], #16
+#ifdef CONFIG_SHADOW_CALL_STACK
+	ldp	scs_sp, xzr, [sp], #16
+#endif
+	ret
+SYM_FUNC_END(call_on_irq_stack)
+NOKPROBE(call_on_irq_stack)
+
 #ifdef CONFIG_ARM_SDE_INTERFACE
 
 #include <asm/sdei.h>