コグノスケ

AArch64の新しい形式のコンテキストスイッチarch_switch() 実装

// zephyr/arch/arm/core/aarch64/switch.S

GTEXT(z_arm64_svc)
SECTION_FUNC(TEXT, z_arm64_svc)
	z_arm64_enter_exc x2, x3, x4    /* ★レジスタをスタックに退避（切り替え前のスレッド）★ */

	switch_el x1, 3f, 2f, 1f
3:
	mrs	x0, esr_el3
	b	0f
2:
	mrs	x0, esr_el2
	b	0f
1:
	mrs	x0, esr_el1
0:
	lsr	x1, x0, #26

	cmp	x1, #0x15 /* 0x15 = SVC */
	bne	inv

	/* Demux the SVC call */
	and	x1, x0, #0xff
	cmp	x1, #_SVC_CALL_CONTEXT_SWITCH
	beq	context_switch    /* ★以下参照★ */

...

context_switch:
	/*
	 * Retrieve x0 and x1 from the stack:
	 *  - x0 = new_thread->switch_handle = switch_to thread
	 *  - x1 = x1 = &old_thread->switch_handle = current thread
	 */
	ldp	x0, x1, [sp, #(16 * 10)]    /* ★x1 = 切り替え前、x0 = 切り替え後のスレッド★ */

	/* Get old thread from x1 */
	sub	x1, x1, ___thread_t_switch_handle_OFFSET

	/* Switch thread */
	bl	z_arm64_context_switch    /* ★コンテキストスイッチ本体（スタックが切り替わる）★ */

exit:
	z_arm64_exit_exc x0, x1, x2    /* ★レジスタをスタックから復旧（切り替え後のスレッド）★ */

...

/**
 * @brief Routine to handle context switches
 *
 * This function is directly called either by _isr_wrapper() in case of
 * preemption, or z_arm64_svc() in case of cooperative switching.
 */

GTEXT(z_arm64_context_switch)
SECTION_FUNC(TEXT, z_arm64_context_switch)
	/* addr of callee-saved regs in thread in x2 */
	ldr	x2, =_thread_offset_to_callee_saved
	add	x2, x2, x1

	/* Store rest of process context including x30 */
	stp	x19, x20, [x2], #16
	stp	x21, x22, [x2], #16
	stp	x23, x24, [x2], #16
	stp	x25, x26, [x2], #16
	stp	x27, x28, [x2], #16
	stp	x29, x30, [x2], #16

	/* Save the current SP */
	mov	x1, sp
	str	x1, [x2]

	/* addr of callee-saved regs in thread in x2 */
	ldr	x2, =_thread_offset_to_callee_saved
	add	x2, x2, x0

	/* Restore x19-x29 plus x30 */
	ldp	x19, x20, [x2], #16
	ldp	x21, x22, [x2], #16
	ldp	x23, x24, [x2], #16
	ldp	x25, x26, [x2], #16
	ldp	x27, x28, [x2], #16
	ldp	x29, x30, [x2], #16

	ldr	x1, [x2]
	mov	sp, x1    /* ★ここで切り替え後のスレッドのスタックに変わる★ */

#ifdef CONFIG_TRACING
	stp	xzr, x30, [sp, #-16]!
	bl	sys_trace_thread_switched_in
	ldp	xzr, x30, [sp], #16
#endif

	/* We restored x30 from the process stack. There are three possible
	 * cases:
	 *
	 * - We return to z_arm64_svc() when swapping in a thread that was
	 *   swapped out by z_arm64_svc() before jumping into
	 *   z_arm64_exit_exc()
	 * - We return to _isr_wrapper() when swapping in a thread that was
	 *   swapped out by _isr_wrapper() before jumping into
	 *   z_arm64_exit_exc()
	 * - We return (jump) into z_thread_entry_wrapper() for new threads
	 *   (see thread.c)
	 */
	ret


// zephyr/include/arm/aarch64/thread.h

struct _callee_saved {
	uint64_t x19;
	uint64_t x20;
	uint64_t x21;
	uint64_t x22;
	uint64_t x23;
	uint64_t x24;
	uint64_t x25;
	uint64_t x26;
	uint64_t x27;
	uint64_t x28;
	uint64_t x29; /* FP */
	uint64_t x30; /* LR */
	uint64_t sp;
};