linux-kernel - Re: [PATCH v2] arm64: Introduce IRQ stack

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Message-ID: <20150917111735.GN25444@e104818-lin.cambridge.arm.com>
Date:	Thu, 17 Sep 2015 12:17:36 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Jungseok Lee <jungseoklee85@...il.com>
Cc:	will.deacon@....com, linux-arm-kernel@...ts.infradead.org,
	mark.rutland@....com, takahiro.akashi@...aro.org,
	James Morse <james.morse@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arm64: Introduce IRQ stack

On Sun, Sep 13, 2015 at 02:42:17PM +0000, Jungseok Lee wrote:
> Currently, kernel context and interrupts are handled using a single
> kernel stack navigated by sp_el1. This forces many systems to use
> 16KB stack, not 8KB one. Low memory platforms naturally suffer from
> memory pressure accompanied by performance degradation.
> 
> This patch addresses the issue as introducing a separate percpu IRQ
> stack to handle both hard and soft interrupts with two ground rules:
> 
>   - Utilize sp_el0 in EL1 context, which is not used currently
>   - Do not complicate current_thread_info calculation
> 
> It is a core concept to trace struct thread_info using sp_el0 instead
> of sp_el1. This approach helps arm64 align with other architectures
> regarding object_is_on_stack() without additional complexity.

I'm still trying to understand how this patch works. I initially thought
that we would set SPSel = 0 while in kernel thread mode to make use of
SP_EL0 but I can't find any such code. Do you still use SP_EL1 all the
time and SP_EL0 just for temporary saving the thread stack?

-- 
Catalin
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