From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756391Ab2LNAgs (ORCPT ); Thu, 13 Dec 2012 19:36:48 -0500 Received: from terminus.zytor.com ([198.137.202.10]:41255 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755594Ab2LNAgr (ORCPT ); Thu, 13 Dec 2012 19:36:47 -0500 Message-ID: <50CA7486.2070804@zytor.com> Date: Thu, 13 Dec 2012 16:36:22 -0800 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0 MIME-Version: 1.0 To: Andy Lutomirski CC: Stefani Seibold , linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de, mingo@redhat.com, ak@linux.intel.com, aarcange@redhat.com, john.stultz@linaro.org Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel References: <1355343572-23074-1-git-send-email-stefani@seibold.net> <50C9148C.4040308@zytor.com> <1355378005.24283.11.camel@wall-e> <1d3061cb-76d0-4e42-9b75-a975b05384ec@email.android.com> <1355379433.24701.1.camel@wall-e> <1355383038.18653.2.camel@wall-e> <50CA6E4C.6000305@zytor.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/13/2012 04:20 PM, Andy Lutomirski wrote: >> >> Whatever data you need you can just map it into the vdso range. There >> really shouldn't be anything special about that at all. >> >> The fixmap stuff is an x86-64 legacy that you don't have to worry about, >> obviously. > > The fixmap page is new. It's not ABI -- the layout can freely change, > but the vdso needs to be able to see. It would make no sense to cow > that page, and it would be more complicated to make it be part of the > (64-bit) vdso, so I left it as a fixmap when I did the vvar cleanups. > Well, the vsyscall fixmap is an ABI. But just because a page is mapped into userspace doesn't mean cow. Think of it as a device mmap, or an mmap of a shared file. > For compat mode, though, I don't think it can be in the fixmap unless > there are segmentation games that I think are impossible, or unless > the vdso wants to do a far call (which I would guess is not much > faster than a system call, although I haven't benchmarked it). This > is because there are no addresses at all that can be seen from 32-bit > code segments and that are in the kernel address range. Yes, you'd have to nip into 64-bit mode which is not really practical. > What you could do is probably arrange (using some linker script magic) > for a symbol to exist that points at the page *before* the vdso > starts. Then just map the vvar page there when starting a compat > task. You could then address it using a normal symbol reference by > tweaking the vvar macro. (I think this'll access it via the GOT.) > Alternatively, you could just pick an absolute address -- the page is > NX, so you don't really need to worry about randomization. IMO it seems this is making it way more complicated than it is. Just make sure you have a section in the vdso where you can map in a data page with the symbols in the right offsets. Extra points for doing magic so that it is at the beginning or end, but I think that might be harder than necessary. -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.