From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754134AbYL0LD6 (ORCPT ); Sat, 27 Dec 2008 06:03:58 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751538AbYL0LDu (ORCPT ); Sat, 27 Dec 2008 06:03:50 -0500 Received: from gw.goop.org ([64.81.55.164]:55856 "EHLO mail.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751500AbYL0LDt (ORCPT ); Sat, 27 Dec 2008 06:03:49 -0500 Message-ID: <49560B87.8090407@goop.org> Date: Sat, 27 Dec 2008 22:03:35 +1100 From: Jeremy Fitzhardinge User-Agent: Thunderbird 2.0.0.18 (X11/20081119) MIME-Version: 1.0 To: Brian Gerst CC: Ingo Molnar , linux-kernel@vger.kernel.org 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> In-Reply-To: <1230052506-5041-2-git-send-email-brgerst@gmail.com> X-Enigmail-Version: 0.95.6 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. J