Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756239AbaKSQl6 (ORCPT ); Wed, 19 Nov 2014 11:41:58 -0500 Received: from mail-la0-f46.google.com ([209.85.215.46]:35515 "EHLO mail-la0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756215AbaKSQlz (ORCPT ); Wed, 19 Nov 2014 11:41:55 -0500 MIME-Version: 1.0 In-Reply-To: References: <20141118172146.3784.81151.stgit@buzz> <20141118173413.GB4042@n2100.arm.linux.org.uk> <546B8C64.3010904@samsung.com> <2041617.Kxhx5O4MMr@wuerfel> <20141119160747.GH4042@n2100.arm.linux.org.uk> Date: Wed, 19 Nov 2014 17:41:53 +0100 Message-ID: Subject: Re: [PATCH RFC] ARM: option for loading modules into vmalloc area From: Ard Biesheuvel To: Nicolas Pitre Cc: Russell King - ARM Linux , Konstantin Khlebnikov , Arnd Bergmann , Andrey Ryabinin , Konstantin Khlebnikov , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19 November 2014 17:37, Nicolas Pitre wrote: > On Wed, 19 Nov 2014, Russell King - ARM Linux wrote: > >> On Wed, Nov 19, 2014 at 05:02:40PM +0100, Ard Biesheuvel wrote: >> > On 19 November 2014 16:52, Konstantin Khlebnikov wrote: >> > > Do you mean ldr pc, =symbol ? >> > > >> > > In this case I get this error: >> > > >> > > /tmp/ccAHtONU.s: Assembler messages: >> > > /tmp/ccAHtONU.s:220: Error: invalid literal constant: pool needs to be closer >> > > >> > > Probably constant pool doesn't work well in inline assembly. >> > > >> > > >> > > Something like this seems work: >> > > >> > > add lr, pc, #4 >> > > ldr pc, [pc, #-4] >> > > .long symbol >> > > >> > >> > You can add a '.ltorg' instruction which tells the assembler to dump >> > the literal pool, but you still need to jump over it, i.e., >> > >> > adr lr, 0f >> > ldr pc, =symbol >> > .ltorg >> > 0: >> >> Which is not a good idea either, because the compiler needs to know how >> far away its own manually generated literal pool is from the instructions >> which reference it. The .ltorg statement can end up emitting any number >> of literals at that point, which makes it indeterminant how many words >> are contained within the asm() statement. >> >> Yes, it isn't desirable to waste an entire data cache line per indirect >> call like the original quote above, but I don't see a practical >> alternative. > > Modules could be built without far calls by default, and then the module > linker would only have to redirect those calls whose destination is too > far away to a dynamically created trampoline table. > > If I remember correctly you even posted some patches to that effect a > couple years ago. Maybe those could be salvaged? > > I would largely recommend a solution where the link process could deal > with it automatically and as needed rather than sprinkling yet more > manually maintained macros into assembly code. > Yes, that would be preferable. I played around with 'bl symbol@PLT' instead of plain 'bl symbol' but unfortunately, our .ko's are not proper ELF shared libraries, so that doesn't work. But essentially, we just need a (eager) PLT. -- Ard. -- 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/