linux-kernel - Re: [PATCH v15 04/10] arm64: Kprobes with single stepping support

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Date:	Wed, 27 Jul 2016 12:38:00 +0100
From:	Dave Martin <Dave.Martin@....com>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Mark Rutland <mark.rutland@....com>,
	Yang Shi <yang.shi@...aro.org>,
	Zi Shen Lim <zlim.lnx@...il.com>,
	Will Deacon <will.deacon@....com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	yalin wang <yalin.wang2010@...il.com>,
	Li Bin <huawei.libin@...wei.com>,
	Jisheng Zhang <jszhang@...vell.com>,
	John Blackwood <john.blackwood@...r.com>,
	Pratyush Anand <panand@...hat.com>,
	David Long <dave.long@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Huang Shijie <shijie.huang@....com>,
	Petr Mladek <pmladek@...e.com>,
	Vladimir Murzin <Vladimir.Murzin@....com>,
	Steve Capper <steve.capper@...aro.org>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Marc Zyngier <marc.zyngier@....com>,
	Mark Brown <broonie@...nel.org>,
	Sandeepa Prabhu <sandeepa.s.prabhu@...il.com>,
	William Cohen <wcohen@...hat.com>,
	Alex Bennée <alex.bennee@...aro.org>,
	Adam Buchbinder <adam.buchbinder@...il.com>,
	linux-arm-kernel@...ts.infradead.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Robin Murphy <robin.murphy@....com>,
	Jens Wiklander <jens.wiklander@...aro.org>,
	Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH v15 04/10] arm64: Kprobes with single stepping support

On Wed, Jul 27, 2016 at 12:19:59PM +0100, Daniel Thompson wrote:

[...]

> It is interesting to note that you force large_func() to stack its arguments
> (by providing 8 dummy int arguments first) then the implicit
> pass-by-reference behavior is still preserved even for a stacked argument;
> large_func() ends up as:
> 
> ~~~
> large_func:
> 	ldr	x0, [sp]
> 	ldr	x0, [x0]
> 	ret
> ~~~
> 
> Only thing is... I *still* haven't found anything in the AArch64 PCS which
> describes this behavior.
> 
> I'm coming to believe that this is a mistake and this information (and the
> threshold at which implicit pass-by-reference kicks in) should be documented
> in section 7.

Is that answered by this?

    B.3. If the argument type is a Composite Type that is larger than
    16 bytes, then the argument is copied to memory allocated by the
    caller and the argument is replaced by a pointer to the copy.

Experimenting with gcc's behaviour seems to back this up.

Cheers
---Dave