From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752280AbYL0LVF (ORCPT ); Sat, 27 Dec 2008 06:21:05 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752894AbYL0LUy (ORCPT ); Sat, 27 Dec 2008 06:20:54 -0500 Received: from gw.goop.org ([64.81.55.164]:48368 "EHLO mail.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752293AbYL0LUx (ORCPT ); Sat, 27 Dec 2008 06:20:53 -0500 Message-ID: <49560F8F.9020901@goop.org> Date: Sat, 27 Dec 2008 22:20:47 +1100 From: Jeremy Fitzhardinge User-Agent: Thunderbird 2.0.0.18 (X11/20081119) MIME-Version: 1.0 To: Ingo Molnar CC: Brian Gerst , linux-kernel@vger.kernel.org, Christoph Lameter , Mike Travis , Thomas Gleixner , "H. Peter Anvin" , Alexander van Heukelum Subject: Re: [PATCH 2/3] x86-64: Unify x86_*_percpu() functions. References: <1230052506-5041-1-git-send-email-brgerst@gmail.com> <1230052506-5041-2-git-send-email-brgerst@gmail.com> <49560B87.8090407@goop.org> <20081227110909.GA15377@elte.hu> In-Reply-To: <20081227110909.GA15377@elte.hu> X-Enigmail-Version: 0.95.6 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Ingo Molnar wrote: > * Jeremy Fitzhardinge wrote: > > >> Brian Gerst wrote: >> >>> Merge the 32-bit and 64-bit versions of these functions. Unlike 32-bit, >>> the segment base is the current cpu's PDA instead of the offset from the >>> original per-cpu area. This is because GCC hardcodes the stackprotector >>> canary at %gs:40. Since the assembler is incapable of relocating against >>> multiple symbols, the code ends up looking like: >>> >>> movq $per_cpu__var, reg >>> subq $per_cpu__pda, reg >>> movq %gs:(reg), reg >>> >>> This is still atomic since the offset is a constant (just calculated at >>> runtime) and not dependant on the cpu number. >>> >>> >> Yeah, it's a real pity we can't convince the linker to do this simple >> computation as a single %gs:ADDR addressing mode. On the other hand, if >> the compiler can reuse the computation of %reg 2-3 times, then the >> generated code could well end up being denser. >> > > There's a nice project for linker hackers? > > I'd like to see some kernel image size measurements done on x86 defconfig > to see how much real impact this has on code density. Unless the impact is > horribly unacceptable, removing ~200 lines of weird x86-specific APIs is a > definitive plus. Yep, I'm all for it. I don't think there'll be much of a size impact at all. J