linux-kernel - Re: [PATCH 4/9] ARM: syscall: always store thread

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Message-ID: <20200928150843.GB1551@shell.armlinux.org.uk>
Date:   Mon, 28 Sep 2020 16:08:43 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Christoph Hellwig <hch@....de>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "linux-kernel@...r.kernel.org" <kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/9] ARM: syscall: always store thread_info->syscall

On Mon, Sep 28, 2020 at 02:42:43PM +0200, Arnd Bergmann wrote:
> > I need some idea how this numberspace is managed in order to
> > understand the code so I can review it, I guess it all makes perfect
> > sense but I need some background here.
> 
> I also had never understood this part before, and I'm still not
> sure where the 0x900000 actually comes from, though my best
> guess is that this was intended as a an OS specific number space,
> with '9' being assigned to Linux (similar to the way Itanium and
> MIPS do with their respective offsets). By the time EABI got added,
> this was apparently no longer considered helpful.

It is an OS specific number space, originally designed to allow
RISC OS programs to be run under Linux.  There was indeed such a
project, but that died and the code ripped out. EABI, by using
SWI 0 - or more accurately, not reading the SWI opcode, trampled
over the ability for RISC OS programs to be run under Linux.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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