linux-kernel - Re: [RFC2 nowrap: PATCH v7 00/18] ILP32 for ARM64

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Date:	Wed, 17 Aug 2016 14:54:59 +0200
From:	"Dr. Philipp Tomsich" <philipp.tomsich@...obroma-systems.com>
To:	Yury Norov <ynorov@...iumnetworks.com>
Cc:	Alexander Graf <agraf@...e.de>, Arnd Bergmann <arnd@...db.de>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
	linux-arch@...r.kernel.org, libc-alpha@...rceware.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	heiko.carstens@...ibm.com, Andrew Pinski <pinskia@...il.com>,
	broonie@...nel.org, "Joseph S. Myers" <joseph@...esourcery.com>,
	Christoph Müllner 
	<christoph.muellner@...obroma-systems.com>,
	"Zhangjian (Bamvor)" <bamvor.zhangjian@...wei.com>,
	szabolcs.nagy@....com, Alexey Klimov <klimov.linux@...il.com>,
	Nathan Lynch <Nathan_Lynch@...tor.com>,
	"Kapoor, Prasun" <Prasun.Kapoor@...iumnetworks.com>,
	kilobyte@...band.pl, geert@...ux-m68k.org,
	manuel.montezelo@...il.com, linyongting@...wei.com,
	maxim.kuvyrkov@...aro.org, davem@...emloft.net,
	zhouchengming1@...wei.com, cmetcalf@...hip.com
Subject: Re: [RFC2 nowrap: PATCH v7 00/18] ILP32 for ARM64


> On 17 Aug 2016, at 14:48, Yury Norov <ynorov@...iumnetworks.com> wrote:
> 
> On Wed, Aug 17, 2016 at 02:28:50PM +0200, Alexander Graf wrote:
>> 
>>> On 17 Aug 2016, at 13:46, Yury Norov <ynorov@...iumnetworks.com> wrote:
>>> 
>>> This series enables aarch64 with ilp32 mode, and as supporting work,
>>> introduces ARCH_32BIT_OFF_T configuration option that is enabled for
>>> existing 32-bit architectures but disabled for new arches (so 64-bit
>>> off_t is is used by new userspace).
>>> 
>>> This version is based on kernel v4.8-rc2.
>>> It works with glibc-2.23, and tested with LTP.
>>> 
>>> This is RFC because there is still no solid understanding what type of registers
>>> top-halves delousing we prefer. In this patchset, w0-w7 are cleared for each
>>> syscall in assembler entry. The alternative approach is in introducing compat
>>> wrappers which is little faster for natively routed syscalls (~2.6% for syscall
>>> with no payload) but much more complicated.
>> 
>> So you’re saying there are 2 options:
>> 
>>  1) easy to get right, slightly slower, same ABI to user space as 2
>>  2) harder to get right, minor performance benefit
> 
> No, ABI is little different. If 1) we pass off_t in a pair to syscalls,
> if 2) - in a single register. So if 1, we 'd take some wrappers from aarch32.
> See patch 12 here.

>From our experience with ILP32, I’d prefer to have off_t (and similar) in a single register
whenever possible (i.e. option #2).  It feels more natural to use the full 64bit registers 
whenever possible, as ILP32 on ARMv8 should really be understood as a 64bit ABI with 
a 32bit memory model.

Cheers,
Philipp.