Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756127AbaKSQhy (ORCPT ); Wed, 19 Nov 2014 11:37:54 -0500 Received: from mail-qa0-f42.google.com ([209.85.216.42]:53645 "EHLO mail-qa0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754398AbaKSQhv (ORCPT ); Wed, 19 Nov 2014 11:37:51 -0500 Date: Wed, 19 Nov 2014 11:37:47 -0500 (EST) From: Nicolas Pitre To: Russell King - ARM Linux cc: Ard Biesheuvel , Konstantin Khlebnikov , Arnd Bergmann , Andrey Ryabinin , Konstantin Khlebnikov , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH RFC] ARM: option for loading modules into vmalloc area In-Reply-To: <20141119160747.GH4042@n2100.arm.linux.org.uk> Message-ID: 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> User-Agent: Alpine 2.11 (LFD 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Nicolas -- 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/