Re: [09/36] AArch64: Exception handling

[Christopher Covington]

Hi Catalin and Will,

On 01/-10/-28163 02:59 PM, Catalin Marinas wrote:
> The patch contains the exception entry code (kernel/entry.S), pt_regs
> structure and related accessors, undefined instruction trapping and
> stack tracing.
> 
> AArch64 Linux kernel (including kernel threads) runs in EL1 mode using
> the SP1 stack. The vectors don't have a fixed address, only alignment
> (2^11) requirements.

[...]

> diff --git a/arch/aarch64/kernel/entry.S b/arch/aarch64/kernel/entry.S
> new file mode 100644
> index 0000000..8ad3995
> --- /dev/null
> +++ b/arch/aarch64/kernel/entry.S
> @@ -0,0 +1,696 @@
> +/*
> + * Low-level exception handling code

[...]

> +/*
> + * Exception vectors.
> + */
> +	.macro	ventry	label
> +	.align	7
> +	b	\label
> +	.endm
> +
> +	.align	11
> +ENTRY(vectors)
> +	ventry	el1_sync_invalid		// Synchronous EL1t
> +	ventry	el1_irq_invalid			// IRQ EL1t
> +	ventry	el1_fiq_invalid			// FIQ EL1t
> +	ventry	el1_error_invalid		// Error EL1t
> +
> +	ventry	el1_sync			// Synchronous EL1h
> +	ventry	el1_irq				// IRQ EL1h
> +	ventry	el1_fiq_invalid			// FIQ EL1h
> +	ventry	el1_error_invalid		// Error EL1h
> +
> +	ventry	el0_sync			// Synchronous 64-bit EL0
> +	ventry	el0_irq				// IRQ 64-bit EL0
> +	ventry	el0_fiq_invalid			// FIQ 64-bit EL0
> +	ventry	el0_error_invalid		// Error 64-bit EL0
> +
> +#ifdef CONFIG_AARCH32_EMULATION
> +	ventry	el0_sync_compat			// Synchronous 32-bit EL0
> +	ventry	el0_irq_compat			// IRQ 32-bit EL0
> +	ventry	el0_fiq_invalid_compat		// FIQ 32-bit EL0
> +	ventry	el0_error_invalid_compat	// Error 32-bit EL0
> +#else
> +	ventry	el0_sync_invalid		// Synchronous 32-bit EL0
> +	ventry	el0_irq_invalid			// IRQ 32-bit EL0
> +	ventry	el0_fiq_invalid			// FIQ 32-bit EL0
> +	ventry	el0_error_invalid		// Error 32-bit EL0
> +#endif
> +END(vectors)
> +
> +/*
> + * Invalid mode handlers
> + */
> +	.macro	inv_entry, el, reason, regsize = 64
> +	kernel_entry el, \regsize
> +	mov	x0, sp
> +	mov	x1, #\reason
> +	mrs	x2, esr_el1
> +	b	bad_mode
> +	.endm

The code seems to indicate that the invalid mode handlers have different alignment requirements than the valid mode handlers, which puzzles me.

> +
> +el0_sync_invalid:
> +	inv_entry 0, BAD_SYNC
> +ENDPROC(el0_sync_invalid)

Plain labels, the ENTRY macro, the END macro and the ENDPROC macro are used variously throughout this file, and I wonder if a greater amount of consistency might be attainable. The description of the ENDPROC macro in include/linux/linkage.h makes me think its use might not be completely warranted in blocks of assembly that don't end with a return instruction.

> +el0_irq_invalid:
> +	inv_entry 0, BAD_IRQ
> +ENDPROC(el0_irq_invalid)
> +
> +el0_fiq_invalid:
> +	inv_entry 0, BAD_FIQ
> +ENDPROC(el0_fiq_invalid)
> +
> +el0_error_invalid:
> +	inv_entry 0, BAD_ERROR
> +ENDPROC(el0_error_invalid)
> +
> +#ifdef CONFIG_AARCH32_EMULATION
> +el0_fiq_invalid_compat:
> +	inv_entry 0, BAD_FIQ, 32
> +ENDPROC(el0_fiq_invalid_compat)
> +
> +el0_error_invalid_compat:
> +	inv_entry 0, BAD_ERROR, 32
> +ENDPROC(el0_error_invalid_compat)
> +#endif
> +
> +el1_sync_invalid:
> +	inv_entry 1, BAD_SYNC
> +ENDPROC(el1_sync_invalid)
> +
> +el1_irq_invalid:
> +	inv_entry 1, BAD_IRQ
> +ENDPROC(el1_irq_invalid)
> +
> +el1_fiq_invalid:
> +	inv_entry 1, BAD_FIQ
> +ENDPROC(el1_fiq_invalid)
> +
> +el1_error_invalid:
> +	inv_entry 1, BAD_ERROR
> +ENDPROC(el1_error_invalid)
> +
> +/*
> + * EL1 mode handlers.
> + */
> +	.align	6
> +el1_sync:
> +	kernel_entry 1
> +	mrs	x1, esr_el1			// read the syndrome register
> +	lsr	x24, x1, #26			// exception class
> +	cmp	x24, #0x25			// data abort in EL1
> +	b.eq	el1_da
> +	cmp	x24, #0x18			// configurable trap
> +	b.eq	el1_undef
> +	cmp	x24, #0x26			// stack alignment exception
> +	b.eq	el1_sp_pc
> +	cmp	x24, #0x22			// pc alignment exception
> +	b.eq	el1_sp_pc
> +	cmp	x24, #0x00			// unknown exception in EL1
> +	b.eq	el1_undef
> +	cmp	x24, #0x30			// debug exception in EL1
> +	b.ge	el1_dbg
> +	b	el1_inv
> +el1_da:
> +	/*
> +	 * Data abort handling
> +	 */
> +	mrs	x0, far_el1
> +	enable_dbg_if_not_stepping x2
> +	// re-enable interrupts if they were enabled in the aborted context
> +	tbnz	x23, #7, 1f			// PSR_I_BIT
> +	enable_irq
> +1:
> +	mov	x2, sp				// struct pt_regs
> +	bl	do_mem_abort
> +
> +	// disable interrupts before pulling preserved data off the stack
> +	disable_irq
> +	kernel_exit 1
> +el1_sp_pc:
> +	/*
> +	 *Stack or PC alignment exception handling
> +	 */
> +	mrs	x0, far_el1
> +	mov	x1, x25
> +	mov	x2, sp
> +	b	do_sp_pc_abort
> +el1_undef:
> +	/*
> +	 *Undefined instruction
> +	 */
> +	mov	x0, sp
> +	b	do_undefinstr
> +el1_dbg:
> +	/*
> +	 * Debug exception handling
> +	 */
> +	tbz	x24, #0, el1_inv		// EL1 only
> +	mrs	x0, far_el1
> +	mov	x2, sp				// struct pt_regs
> +	bl	do_debug_exception
> +
> +	kernel_exit 1
> +el1_inv:
> +	// TODO: add support for undefined instructions in kernel mode
> +	mov	x0, sp
> +	mov	x1, #BAD_SYNC
> +	mrs	x2, esr_el1
> +	b	bad_mode
> +ENDPROC(el1_sync)
> +
> +	.align	6
> +el1_irq:
> +	kernel_entry 1
> +	enable_dbg_if_not_stepping x0
> +#ifdef CONFIG_TRACE_IRQFLAGS
> +	bl	trace_hardirqs_off
> +#endif
> +#ifdef CONFIG_PREEMPT
> +	get_thread_info tsk
> +	ldr	x24, [tsk, #TI_PREEMPT]		// get preempt count
> +	add	x0, x24, #1			// increment it
> +	str	x0, [tsk, #TI_PREEMPT]
> +#endif
> +	irq_handler
> +#ifdef CONFIG_PREEMPT
> +	str	x24, [tsk, #TI_PREEMPT]		// restore preempt count
> +	cbnz	x24, 1f				// preempt count != 0
> +	ldr	x0, [tsk, #TI_FLAGS]		// get flags
> +	tbz	x0, #TIF_NEED_RESCHED, 1f	// needs rescheduling?
> +	bl	el1_preempt
> +1:
> +#endif
> +#ifdef CONFIG_TRACE_IRQFLAGS
> +	bl	trace_hardirqs_on
> +#endif
> +	kernel_exit 1
> +ENDPROC(el1_irq)
> +
> +#ifdef CONFIG_PREEMPT
> +el1_preempt:
> +	mov	x24, lr
> +1:	enable_dbg
> +	bl	preempt_schedule_irq		// irq en/disable is done inside
> +	ldr	x0, [tsk, #TI_FLAGS]		// get new tasks TI_FLAGS
> +	tbnz	x0, #TIF_NEED_RESCHED, 1b	// needs rescheduling?
> +	ret	x24
> +#endif
> +
> +/*
> + * EL0 mode handlers.
> + */
> +	.align	6
> +el0_sync:
> +	kernel_entry 0
> +	mrs	x25, esr_el1			// read the syndrome register
> +	lsr	x24, x25, #26			// exception class
> +	cmp	x24, #0x15			// SVC in 64-bit state
> +	b.eq	el0_svc
> +	adr	lr, ret_from_exception
> +	cmp	x24, #0x24			// data abort in EL0
> +	b.eq	el0_da
> +	cmp	x24, #0x20			// instruction abort in EL0
> +	b.eq	el0_ia
> +	cmp	x24, #0x07			// FP/ASIMD access
> +	b.eq	el0_fpsimd_acc
> +	cmp	x24, #0x2c			// FP/ASIMD exception
> +	b.eq	el0_fpsimd_exc
> +	cmp	x24, #0x18			// configurable trap
> +	b.eq	el0_undef
> +	cmp	x24, #0x26			// stack alignment exception
> +	b.eq	el0_sp_pc
> +	cmp	x24, #0x22			// pc alignment exception
> +	b.eq	el0_sp_pc
> +	cmp	x24, #0x00			// unknown exception in EL0
> +	b.eq	el0_undef
> +	cmp	x24, #0x30			// debug exception in EL0
> +	b.ge	el0_dbg
> +	b	el0_inv
> +
> +#ifdef CONFIG_AARCH32_EMULATION
> +	.align	6
> +el0_sync_compat:
> +	kernel_entry 0, 32
> +	mrs	x25, esr_el1			// read the syndrome register
> +	lsr	x24, x25, #26			// exception class
> +	cmp	x24, #0x11			// SVC in 32-bit state
> +	b.eq	el0_svc_compat
> +	adr	lr, ret_from_exception
> +	cmp	x24, #0x24			// data abort in EL0
> +	b.eq	el0_da
> +	cmp	x24, #0x20			// instruction abort in EL0
> +	b.eq	el0_ia
> +	cmp	x24, #0x07			// FP/ASIMD access
> +	b.eq	el0_fpsimd_acc
> +	cmp	x24, #0x28			// FP/ASIMD exception
> +	b.eq	el0_fpsimd_exc
> +	cmp	x24, #0x00			// unknown exception in EL0
> +	b.eq	el0_undef
> +	cmp	x24, #0x30			// debug exception in EL0
> +	b.ge	el0_dbg
> +	b	el0_inv
> +el0_svc_compat:
> +	/*
> +	 * AArch32 syscall handling
> +	 */
> +	adr	stbl, compat_sys_call_table	// load compat syscall table pointer
> +	uxtw	scno, w7			// syscall number in w7 (r7)
> +	mov     sc_nr, #__NR_compat_syscalls
> +	b	el0_svc_naked
> +
> +	.align	6
> +el0_irq_compat:
> +	kernel_entry 0, 32
> +	b	el0_irq_naked
> +#endif
> +
> +el0_da:
> +	/*
> +	 * Data abort handling
> +	 */
> +	mrs	x0, far_el1
> +	disable_step x1
> +	isb
> +	enable_dbg
> +	// enable interrupts before calling the main handler
> +	enable_irq
> +	mov	x1, x25
> +	mov	x2, sp
> +	b	do_mem_abort
> +el0_ia:
> +	/*
> +	 * Instruction abort handling
> +	 */
> +	mrs	x0, far_el1
> +	disable_step x1
> +	isb
> +	enable_dbg
> +	// enable interrupts before calling the main handler
> +	enable_irq
> +	orr	x1, x25, #1 << 24		// use reserved ISS bit for instruction aborts
> +	mov	x2, sp
> +	b	do_mem_abort
> +el0_fpsimd_acc:
> +	/*
> +	 * Floating Point or Advanced SIMD access
> +	 */
> +	mov	x0, x25
> +	mov	x1, sp
> +	b	do_fpsimd_acc
> +el0_fpsimd_exc:
> +	/*
> +	 * Floating Point or Advanced SIMD exception
> +	 */
> +	mov	x0, x25
> +	mov	x1, sp
> +	b	do_fpsimd_exc
> +el0_sp_pc:
> +	/*
> +	 * Stack or PC alignment exception handling
> +	 */
> +	mrs	x0, far_el1
> +	disable_step x1
> +	isb
> +	enable_dbg
> +	// enable interrupts before calling the main handler
> +	enable_irq
> +	mov	x1, x25
> +	mov	x2, sp
> +	b	do_sp_pc_abort
> +el0_undef:
> +	/*
> +	 *Undefined instruction
> +	 */
> +	mov	x0, sp
> +	b	do_undefinstr
> +el0_dbg:
> +	/*
> +	 * Debug exception handling
> +	 */
> +	tbnz	x24, #0, el0_inv		// EL0 only
> +	mrs	x0, far_el1
> +	disable_step x1
> +	mov	x1, x25
> +	mov	x2, sp
> +	b	do_debug_exception
> +el0_inv:
> +	mov	x0, sp
> +	mov	x1, #BAD_SYNC
> +	mrs	x2, esr_el1
> +	b	bad_mode
> +ENDPROC(el0_sync)
> +
> +	.align	6
> +el0_irq:
> +	kernel_entry 0
> +el0_irq_naked:
> +	disable_step x1
> +	isb
> +	enable_dbg
> +#ifdef CONFIG_TRACE_IRQFLAGS
> +	bl	trace_hardirqs_off
> +#endif
> +	get_thread_info tsk
> +#ifdef CONFIG_PREEMPT
> +	ldr	x24, [tsk, #TI_PREEMPT]		// get preempt count
> +	add	x23, x24, #1			// increment it
> +	str	x23, [tsk, #TI_PREEMPT]
> +#endif
> +	irq_handler
> +#ifdef CONFIG_PREEMPT
> +	ldr	x0, [tsk, #TI_PREEMPT]
> +	str	x24, [tsk, #TI_PREEMPT]
> +	cmp	x0, x23
> +	b.eq	1f
> +	mov	x1, #0
> +	str	x1, [x1]			// BUG

It looks like the error handling here isn't quite complete.

> +1:
> +#endif
> +#ifdef CONFIG_TRACE_IRQFLAGS
> +	bl	trace_hardirqs_on
> +#endif
> +	b	ret_to_user
> +ENDPROC(el0_irq)

[...]

Regards,
Christopher

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