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Subject: Re: [PATCH v6 12/20] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it
From: "Dr. Philipp Tomsich" <philipp.tomsich@theobroma-systems.com>
In-Reply-To: <2334903.IjsQ1BK3JF@wuerfel>
Date: Mon, 21 Dec 2015 19:39:35 +0100
Cc: Catalin Marinas <catalin.marinas@arm.com>,
        Andrew Pinski <pinskia@gmail.com>,
        "Joseph S. Myers" <joseph@codesourcery.com>,
        "Kapoor, Prasun" <Prasun.Kapoor@caviumnetworks.com>,
        broonie@kernel.org, Nathan Lynch <Nathan_Lynch@mentor.com>,
        LKML <linux-kernel@vger.kernel.org>, Alexander Graf <agraf@suse.de>,
        Alexey Klimov <klimov.linux@gmail.com>,
        Yury Norov <ynorov@caviumnetworks.com>,
        Jan Dakinevich <jan.dakinevich@gmail.com>,
        Andrew Pinski <apinski@cavium.com>,
        David Daney <ddaney.cavm@gmail.com>, Andreas Schwab <schwab@suse.de>,
        "Zhangjian (Bamvor)" <bamvor.zhangjian@huawei.com>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        =?utf-8?Q?Christoph_M=C3=BCllner?= 
	<christoph.muellner@theobroma-systems.com>,
        Marcus Shawcroft <Marcus.Shawcroft@arm.com>
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To: Arnd Bergmann <arnd@arndb.de>
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> On 18 Dec 2015, at 13:47, Arnd Bergmann <arnd@arndb.de> wrote:
> 
>> 3. Follow the PCS up to glibc but always pass syscall arguments in W
>>   registers, like AArch32 compat support (the least preferred option,
>>   the only advantage is a single wrapper for all syscalls but it would
>>   be doing unnecessary zeroing even for syscalls where it isn't needed)
> 
> This would mean we cannot pass 64-bit arguments in registers, right?

Note that there’s no 32bit registers (the ‘w’-form always refers to the lower
32bits of a 64bit register, with implicit zero-extension)… and load/store
instructions always use the full base-register (‘x’-form) for address calculation.
I.e. a load/store would inadvertently pickup “random garbage” in the upper 
32bits, if no explicit zero-extension is applied.

In other words: all zero-extensions for 32bit arguments should be explicit
on the kernel side.

Regards,
Philipp.

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