Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757654AbYFDNso (ORCPT ); Wed, 4 Jun 2008 09:48:44 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752837AbYFDNsh (ORCPT ); Wed, 4 Jun 2008 09:48:37 -0400 Received: from relay2.sgi.com ([192.48.171.30]:38399 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752262AbYFDNsg (ORCPT ); Wed, 4 Jun 2008 09:48:36 -0400 Message-ID: <48469D30.40901@sgi.com> Date: Wed, 04 Jun 2008 06:48:32 -0700 From: Mike Travis User-Agent: Thunderbird 2.0.0.6 (X11/20070801) MIME-Version: 1.0 To: Jeremy Fitzhardinge CC: Ingo Molnar , Andrew Morton , Christoph Lameter , David Miller , Eric Dumazet , linux-kernel@vger.kernel.org, Rusty Russell Subject: Re: [PATCH 3/4] x86_64: Fold pda into per cpu area References: <20080604003018.538497000@polaris-admin.engr.sgi.com> <20080604003019.509483000@polaris-admin.engr.sgi.com> <484691CB.6090809@goop.org> In-Reply-To: <484691CB.6090809@goop.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2467 Lines: 73 Jeremy Fitzhardinge wrote: > Mike Travis wrote: >> * Declare the pda as a per cpu variable. >> >> * Make the x86_64 per cpu area start at zero. >> >> * Since the pda is now the first element of the per_cpu area, cpu_pda() >> is no longer needed and per_cpu() can be used instead. This also >> makes >> the _cpu_pda[] table obsolete. >> >> * Since %gs is pointing to the pda, it will then also point to the >> per cpu >> variables and can be accessed thusly: >> >> %gs:[&per_cpu_xxxx - __per_cpu_start] >> The above is only a partial story (I folded the two patches but didn't update the comments correctly.] The variables are already offset from __per_cpu_start by virtue of the .data.percpu section being based at zero. Therefore only the %gs register needs to be set to the base of each cpu's percpu section to resolve the target address: %gs:&per_cpu_xxxx And the .data.percpu.first forces the pda percpu variable to the front. > > Unfortunately that doesn't actually work, because you can't have a reloc > with two variables. > > In something like: > > mov %gs:per_cpu__foo - 12345, %rax > mov %gs:per_cpu__foo, %rax > mov %gs:per_cpu__foo - 12345(%rip), %rax > mov %gs:per_cpu__foo(%rip), %rax > mov %gs:per_cpu__foo - __per_cpu_start, %rax > mov %gs:per_cpu__foo - __per_cpu_start(%rip), %rax > > the last two lines will not assemble: > > t.S:5: Error: can't resolve `per_cpu__foo' {*UND* section} - > `__per_cpu_start' {*UND* section} > t.S:6: Error: can't resolve `per_cpu__foo' {*UND* section} - > `__per_cpu_start' {*UND* section} > > Unfortunately, the only way I can think of fixing this is to compute the > offset into a temp register, then use that: > > lea per_cpu__foo(%rip), %rax > mov %gs:__per_cpu_offset(%rax), %rax > > (where __per_cpu_offset is defined in the linker script as > -__per_cpu_start). > > This seems to be a general problem with zero-offset per-cpu. And its > unfortunate, because no-register access to per-cpu variables is nice to > have. > > The other alternative - and I have no idea whether this is practical or > possible - is to define a complete set of pre-offset per_cpu symbols. > > J -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/