linux-kernel - Re: [PATCH 01/23] all: syscall wrappers: add documentation

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Date:	Wed, 25 May 2016 23:03:27 +0300
From:	Yury Norov <ynorov@...iumnetworks.com>
To:	David Miller <davem@...emloft.net>
CC:	<arnd@...db.de>, <catalin.marinas@....com>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-arch@...r.kernel.org>, <linux-s390@...r.kernel.org>,
	<libc-alpha@...rceware.org>, <schwidefsky@...ibm.com>,
	<heiko.carstens@...ibm.com>, <pinskia@...il.com>,
	<broonie@...nel.org>, <joseph@...esourcery.com>,
	<christoph.muellner@...obroma-systems.com>,
	<bamvor.zhangjian@...wei.com>, <szabolcs.nagy@....com>,
	<klimov.linux@...il.com>, <Nathan_Lynch@...tor.com>,
	<agraf@...e.de>, <Prasun.Kapoor@...iumnetworks.com>,
	<kilobyte@...band.pl>, <geert@...ux-m68k.org>,
	<philipp.tomsich@...obroma-systems.com>
Subject: Re: [PATCH 01/23] all: syscall wrappers: add documentation

On Wed, May 25, 2016 at 12:30:17PM -0700, David Miller wrote:
> From: Yury Norov <ynorov@...iumnetworks.com>
> Date: Tue, 24 May 2016 03:04:30 +0300
> 
> > +To clear that top halves, automatic wrappers are introduced. They clear all
> > +required registers before passing control to regular syscall handler.
> 
> Why have one of these for every single compat system call, rather than
> simply clearing the top half of all of these registers unconditionally
> in the 32-bit system call trap before the system call is invoked?
> 
> That's what we do on sparc64.
> 
> And with that, you only need wrappers for the case where there needs
> to be proper sign extention of a 32-bit signed argument.

It was discussed as one of possible solutions. The downside of it is
that we cannot pass 64-bit types (like off_t) in single register.
The other downside is that we clear top halves for every single
syscall, and it looks excessive. So, from spark64 and s390 approaches
we choosed second.