linux-kernel - Re: [PATCH v5] arm64: Introduce IRQ stack

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Date:	Tue, 20 Oct 2015 17:04:22 +0100
From:	James Morse <james.morse@....com>
To:	Jungseok Lee <jungseoklee85@...il.com>
CC:	catalin.marinas@....com, will.deacon@....com,
	linux-arm-kernel@...ts.infradead.org, takahiro.akashi@...aro.org,
	mark.rutland@....com, barami97@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] arm64: Introduce IRQ stack

On 20/10/15 16:05, Jungseok Lee wrote:
> On Oct 20, 2015, at 7:05 PM, James Morse wrote:
>> On 17/10/15 15:27, Jungseok Lee wrote:
>>> diff --git a/arch/arm64/kernel/irq.c b/arch/arm64/kernel/irq.c
>>> index 9f17ec0..13fe8f4 100644
>>> --- a/arch/arm64/kernel/irq.c
>>> +++ b/arch/arm64/kernel/irq.c
>>> @@ -30,6 +30,8 @@
>>>
>>> unsigned long irq_err_count;
>>>
>>> +DEFINE_PER_CPU(void *, irq_stacks);
>>> +
>>> int arch_show_interrupts(struct seq_file *p, int prec)
>>> {
>>> 	show_ipi_list(p, prec);
>>> @@ -47,9 +49,31 @@ void __init set_handle_irq(void (*handle_irq)(struct pt_regs *))
>>> 	handle_arch_irq = handle_irq;
>>> }
>>>
>>> +static char boot_irq_stack[IRQ_STACK_SIZE] __aligned(IRQ_STACK_SIZE);
>>
>> Is kmalloc() not available this early? Regardless:
>> As Catalin is happy with the Image size increase [0], this could become
>> something like:
>>> DEFINE_PER_CPU(union thread_union, irq_stack);
>> Which will remove the need to __alloc_irq_stack()s.
> 
> We cannot rely on static allocation using percpu in case of 4KB page system.
> Since ARM64 utilizes generic setup_per_cpu_areas(), tpidr_el1 is PAGE_SIZE
> aligned. That is, IRQ stack is allocated with PAGE_SIZE alignment for secondary
> cores. However, the top-bit method works well under the assumption that IRQ
> stack is IRQ_STACK_SIZE aligned. It leads to IRQ re-entrance check failure.

Yikes! That is nasty... well caught!

Now I understand why you had the per-cpu version #ifdef'd in your example
hunk earlier!

Do we need the irq stack to be aligned like this? It was originally for the
old implementation of current_thread_info(), which this patch changes.

If its just the re-entrance check that needs the alignment, maybe the
irq_count approach is better (but count late not early), and drop the
alignment requirement on interrupt stacks. We know re-entrant irqs will
keep sp_el0, so the new current_thread_info() still works.

I think Catalin's comment is to count like x86 (64 bit version) does in
arch/x86/entry/entry_64.S:do_softirq_own_stack, and treat this as a
re-entrance flag in entry.S.

task stacks still need to be aligned, as when user space is interrupted, we
have a kernel stack, and no idea where its struct task_struct is, unless we
know it was originally THREAD_SIZE aligned.

James

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