From mboxrd@z Thu Jan  1 00:00:00 1970
From: arnd@arndb.de (Arnd Bergmann)
Date: Fri, 27 May 2016 10:42:59 +0200
Subject: [PATCH 01/23] all: syscall wrappers: add documentation
In-Reply-To: <20160527060357.GB3820@osiris>
References: <6293194.tGy03QJ9ME@wuerfel> <20160527003753.GA14247@yury-N73SV>
 <20160527060357.GB3820@osiris>
Message-ID: <13240365.okADkKsTBJ@wuerfel>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Friday, May 27, 2016 8:03:57 AM CEST Heiko Carstens wrote:
> > > > > Cost wise, this seems like it all cancels out in the end, but what
> > > > > do I know?
> > > > 
> > > > I think you know something, and I also think Heiko and other s390 guys
> > > > know something as well. So I'd like to listen their arguments here.
> 
> If it comes to 64 bit arguments for compat system calls: s390 also has an
> x32-like ABI extension which allows user space to use full 64 bit
> registers. As far as I know hardly anybody ever made use of that.
> 
> However even if that would be widely used, to me it wouldn't make sense to
> add new compat system calls which allow 64 bit arguments, simply because
> something like
> 
> c = (u32)a | (u64)b << 32;
> 
> can be done with a single 1-cycle instruction. It's just not worth the
> extra effort to maintain additional system call variants.

For reference, both tile and mips also have separate 32-bit ABIs that are
only used on 64-bit kernels (aside from the normal 32-bit ABI). Tile
does it like s390 and passes 64-bit arguments as pairs, while MIPS
and x86 and pass them as single registers.

Tile is very similar to arm64 because it also uses the generic system
call table, which I think is a good argument to keep them in sync.

	Arnd