From mboxrd@z Thu Jan 1 00:00:00 1970 From: Catalin Marinas Subject: Re: [PATCH 01/23] all: syscall wrappers: add documentation Date: Fri, 27 May 2016 10:01:42 +0100 Message-ID: <20160527090141.GA7865@e104818-lin.cambridge.arm.com> References: <6293194.tGy03QJ9ME@wuerfel> <20160525.135039.244098606649448826.davem@davemloft.net> <6407614.fdv5XFSBue@wuerfel> <20160525.142821.1719403997976778673.davem@davemloft.net> <20160526204819.GA10274@yury-N73SV> <20160526222943.GA16729@MBP.local> <20160527003753.GA14247@yury-N73SV> <20160527060357.GB3820@osiris> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from foss.arm.com ([217.140.101.70]:43229 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932235AbcE0JBt (ORCPT ); Fri, 27 May 2016 05:01:49 -0400 Content-Disposition: inline In-Reply-To: <20160527060357.GB3820@osiris> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Heiko Carstens Cc: Yury Norov , David Miller , arnd@arndb.de, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, libc-alpha@sourceware.org, schwidefsky@de.ibm.com, pinskia@gmail.com, broonie@kernel.org, joseph@codesourcery.com, christoph.muellner@theobroma-systems.com, bamvor.zhangjian@huawei.com, szabolcs.nagy@arm.com, klimov.linux@gmail.com, Nathan_Lynch@mentor.com, agraf@suse.de, Prasun.Kapoor@caviumnetworks.com, kilobyte@angband.pl, geert@linux-m68k.org, philipp.tomsich@theobroma-systems.com On Fri, May 27, 2016 at 08:03:57AM +0200, Heiko Carstens wrote: > > > > The cost is pretty trivial though. See kernel/compat_wrapper.o: > > > > COMPAT_SYSCALL_WRAP2(creat, const char __user *, pathname, umode_t, mode); > > > > 0: a9bf7bfd stp x29, x30, [sp,#-16]! > > > > 4: 910003fd mov x29, sp > > > > 8: 2a0003e0 mov w0, w0 > > > > c: 94000000 bl 0 > > > > 10: a8c17bfd ldp x29, x30, [sp],#16 > > > > 14: d65f03c0 ret > > > > > > I would say the above could be more expensive than 8 movs (16 bytes to > > > write, read, a branch and a ret). You can also add the I-cache locality, > > > having wrappers for each syscalls instead of a single place for zeroing > > > the upper half (where no other wrapper is necessary). > > > > > > Can we trick the compiler into doing a tail call optimisation. This > > > could have simply been: > > > > > > COMPAT_SYSCALL_WRAP2(creat, ...): > > > mov w0, w0 > > > b > > > > What you talk about was in my initial version. But Heiko insisted on having all > > wrappers together. > > http://www.spinics.net/lists/linux-s390/msg11593.html > > > > Grep your email for discussion. > > I think Catalin's question was more about why there is even a stack frame > generated. It looks like it is not necessary. I did ask this too a couple > of months ago, when we discussed this. Indeed, I was questioning the need for prologue/epilogue, not the use of COMPAT_SYSCALL_WRAPx(). Maybe something like __naked would do. > > > > > 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. If we split 64-bit arguments in two, we can go a step further and avoid most of the COMPAT_SYSCALL_WRAPx annotations in favour of a common upper-half zeroing of the argument registers on ILP32 syscall entry. There would be a few exceptions where we need to re-build 64-bit arguments on sign-extend. -- Catalin