From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755923Ab2LNCSq (ORCPT ); Thu, 13 Dec 2012 21:18:46 -0500 Received: from terminus.zytor.com ([198.137.202.10]:41899 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753444Ab2LNCSp (ORCPT ); Thu, 13 Dec 2012 21:18:45 -0500 User-Agent: K-9 Mail for Android In-Reply-To: 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> <50CA81A4.9040702@zytor.com> <50CA85BD.7070502@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Subject: Re: [PATCH] Add VDSO time function support for x86 32-bit kernel From: "H. Peter Anvin" Date: Thu, 13 Dec 2012 18:18:05 -0800 To: Andy Lutomirski , criu@openvz.org 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 Message-ID: <8c3585bc-fc7d-4826-913c-f4581494d91d@email.android.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Wouldn't the vdso get mapped already and could be mremap()'d. If we really need more control I'd almost push for a device/filesystem node that could be mmapped the usual way. Andy Lutomirski wrote: >On Thu, Dec 13, 2012 at 5:49 PM, H. Peter Anvin wrote: >> On 12/13/2012 05:42 PM, Andy Lutomirski wrote: >>> >>> The 64-bit/x32 case is currently very simple and fast because it >uses >>> absolute addressing. Admittedly, pcrel references are free, so >>> changing this wouldn't cost much. For native, it'll be slower, but >>> maybe no one cares. I seem to care about this more than anyone >else, >>> and I don't use 32 bit code. :) >>> >> >> pcrel is actually cheaper than absolute addressing in 64-bit mode. >> >>> The benefit of switching is that the vdso code could be the same in >>> all three cases. (Actually, it's even better than that. All of the >>> VVAR magic could be the same in the vdso and the kernel -- the >kernel >>> linker script would just have to have an appropriate symbol to see >the >>> appropriate mapping.) >>> >>> >>> This: >>> >>> __attribute__((visibility("hidden"))) int foo; >>> >>> int get_foo(void) >>> { >>> return foo; >>> } >>> >>> generates a rip-relative access on 64 bits and GOTOFF on 32 bits. >>> >>> The only reason I didn't use a real symbol in the first place is >>> because I couldn't figure out how to get gcc to emit an absolute >>> relocation in pic code. >> >> Well, then, we wouldn't need to do that... this is starting to sound >> like a significant win. > >How will this avoid breaking checkpoint/restore in userspace? If the >vdso is not just plain old code, criu presumably needs to know about >it. Should there be an arch_prctl(ARCH_MAP_VDSO, addr) to create a >vdso mapping somewhere? > >--Andy -- Sent from my mobile phone. Please excuse brevity and lack of formatting.