Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755747AbaKSQcb (ORCPT ); Wed, 19 Nov 2014 11:32:31 -0500 Received: from mail-wi0-f181.google.com ([209.85.212.181]:41215 "EHLO mail-wi0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753832AbaKSQca (ORCPT ); Wed, 19 Nov 2014 11:32:30 -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 20:32:27 +0400 Message-ID: Subject: Re: [PATCH RFC] ARM: option for loading modules into vmalloc area From: Konstantin Khlebnikov To: Ard Biesheuvel Cc: Russell King - ARM Linux , 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 Wed, Nov 19, 2014 at 7:25 PM, Ard Biesheuvel wrote: > On 19 November 2014 17:07, 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. >> > > That applies to any inline asm statement in general: the compiler > assumes that the expanded size will not interfere with its ability to > emit literals after the function's return instruction. > Sometimes it will put a literal pool in the middle of the function if > it is very large, and I am not sure if an inline asm by itself would > ever trigger that heuristic to kick in. > > But by the same logic, i.e., due to the fact that GCC manages its own > literals, the literal pool at the assembly level is unlikely to be so > large that you will actually hit this condition. > >> 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. >> > > We could at least add some labels instead of doing explicit pc arithmetic, i.e., > > adr lr, 1f > ldr pc, 0f > 0: .long symbol > 1: I think we need some unique prefix here, this macro is used inside bigger inline assembly constructions and probably another macro. -- 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/