From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1755545AbcG0Lm7 (ORCPT <rfc822;w@1wt.eu>);
	Wed, 27 Jul 2016 07:42:59 -0400
Received: from mail-wm0-f41.google.com ([74.125.82.41]:38746 "EHLO
	mail-wm0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753636AbcG0Lm6 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 27 Jul 2016 07:42:58 -0400
Subject: Re: [PATCH v15 04/10] arm64: Kprobes with single stepping support
To: Dave Martin <Dave.Martin@arm.com>
References: <578FA238.3050206@arm.com> <5790F960.5050007@linaro.org>
 <57910528.7070902@arm.com> <57911590.50305@linaro.org>
 <20160722101617.GA17821@e104818-lin.cambridge.arm.com>
 <57924104.1080202@linaro.org>
 <20160725171350.GE2423@e104818-lin.cambridge.arm.com>
 <ce78f577-d14a-7b12-c96f-a6ebd8866283@linaro.org>
 <20160726175447.GA28376@leverpostej>
 <a90e1e01-b0c8-f57b-ada5-835f9d5736bf@linaro.org>
 <20160727113759.GB7147@e103592.cambridge.arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>, Yang Shi <yang.shi@linaro.org>,
        Zi Shen Lim <zlim.lnx@gmail.com>, Will Deacon <will.deacon@arm.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        yalin wang <yalin.wang2010@gmail.com>,
        Li Bin <huawei.libin@huawei.com>, Jisheng Zhang <jszhang@marvell.com>,
        John Blackwood <john.blackwood@ccur.com>,
        Pratyush Anand <panand@redhat.com>, David Long <dave.long@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Huang Shijie <shijie.huang@arm.com>, Petr Mladek <pmladek@suse.com>,
        Vladimir Murzin <Vladimir.Murzin@arm.com>,
        Steve Capper <steve.capper@linaro.org>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Marc Zyngier <marc.zyngier@arm.com>, Mark Brown <broonie@kernel.org>,
        Sandeepa Prabhu <sandeepa.s.prabhu@gmail.com>,
        William Cohen <wcohen@redhat.com>,
        =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>,
        Adam Buchbinder <adam.buchbinder@gmail.com>,
        linux-arm-kernel@lists.infradead.org,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        linux-kernel@vger.kernel.org, James Morse <james.morse@arm.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Jens Wiklander <jens.wiklander@linaro.org>,
        Christoffer Dall <christoffer.dall@linaro.org>
From: Daniel Thompson <daniel.thompson@linaro.org>
Message-ID: <0bb682ba-aede-97a3-c858-4cd1c28fd68b@linaro.org>
Date: Wed, 27 Jul 2016 12:42:53 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
 Thunderbird/45.1.1
MIME-Version: 1.0
In-Reply-To: <20160727113759.GB7147@e103592.cambridge.arm.com>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 27/07/16 12:38, Dave Martin wrote:
> 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.

Absolutely answered by that. Thanks (and sorry for the noise)!


Daniel.