From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756301Ab2LNAUg (ORCPT ); Thu, 13 Dec 2012 19:20:36 -0500 Received: from mail-vc0-f174.google.com ([209.85.220.174]:51881 "EHLO mail-vc0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753580Ab2LNAUf (ORCPT ); Thu, 13 Dec 2012 19:20:35 -0500 MIME-Version: 1.0 In-Reply-To: <50CA6E4C.6000305@zytor.com> 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> From: Andy Lutomirski Date: Thu, 13 Dec 2012 16:20:14 -0800 Message-ID: Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel To: "H. Peter Anvin" 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 Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 13, 2012 at 4:09 PM, H. Peter Anvin wrote: > On 12/13/2012 11:32 AM, Andy Lutomirski wrote: >> >> >> x32's vdso cheats -- x32 code can see high addresses just fine. The >> toolchain just makes it difficult. >> >> Your best bet is probably to just map the vvar page twice -- once at >> the same address as native 32-bit mode (but only for compat tasks) >> would use and once in the usual fixmap location. You can't use the >> fixmap for the compat mapping, though, since it would be a *user* >> address. >> >> For HPET support, you'd have to have special support. I'd say to skip >> it for compat mode. >> > > 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. 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. 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. --Andy