Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753817AbZGJHYJ (ORCPT ); Fri, 10 Jul 2009 03:24:09 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751745AbZGJHXw (ORCPT ); Fri, 10 Jul 2009 03:23:52 -0400 Received: from vpn.id2.novell.com ([195.33.99.129]:32305 "EHLO vpn.id2.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751843AbZGJHXu convert rfc822-to-8bit (ORCPT ); Fri, 10 Jul 2009 03:23:50 -0400 Message-Id: <4A57089B0200007800009C0E@vpn.id2.novell.com> X-Mailer: Novell GroupWise Internet Agent 8.0.0 Date: Fri, 10 Jul 2009 08:23:39 +0100 From: "Jan Beulich" To: "Rusty Russell" , "Michal Marek" Cc: "Ingo Molnar" , , "Jon Masters" , "Thomas Gleixner" , , , , Subject: Re: [PATCH] reduce export symbol CRC table size on 64-bit archs References: <4A4A18780200007800008345@vpn.id2.novell.com> <4A4E671C.2090201@suse.cz> <4A51C71B0200007800008EE2@vpn.id2.novell.com> <200907092044.22108.rusty@rustcorp.com.au> In-Reply-To: <200907092044.22108.rusty@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2910 Lines: 71 >>> Rusty Russell 09.07.09 13:14 >>> >On Mon, 6 Jul 2009 05:12:51 pm Jan Beulich wrote: >> >Jan Beulich napsal(a): >> >> Since these CRCs are really only 32-bit quantities, there's no need to >> >> store them in 64-bit slots. Since, however, gcc doesn't allow >> >> respective initializations, asm() constructs get used to create the CRC >> >> tables (and its for that reason that the patch only makes x86-64 and >> >> ia64 utilize that functionality, as I can't verify this doesn't break >> >> in some subtle way elsewhere). >> > >> >... >> > >> >> struct modversion_info >> >> { >> >> - unsigned long crc; >> >> + ksym_crc_t crc; >> >> char name[MODULE_NAME_LEN]; >> >> }; >> > >> >This change breaks module-init-tools: >> >Before: >> >$ /sbin/modprobe --dump-modversions _build/drivers/usb/core/usbcore.ko >> > >> >| head >> > >> >0xb49b735a module_layout >> >0xdb7e6a70 bus_register >> >... >> >After: >> >$ /sbin/modprobe --dump-modversions >> >_build-crc-int/drivers/usb/core/usbcore.ko | head >> >0x75646f6d91ea7b5c le_layout >> >0x5f7375623e215f43 register >> >... >> >It also breaks the newly added depmod -E option (check symbol versions), >> >which also reads the struct modversion_info array (*). Is it possible >> >name the section differently (__versions2?) on those architectures where >> >the size changes, so that it is possible to fix m-i-t in a >> >backwards-compatible manner? >> >> First of all I'd view it as a design bug if user mode code assumptions >> prevent changes to the kernel. > >Yes, but unfortunately it happens. We do it much less than we used to, but >there are limits. > >> But taking this as an uncorrectable fact, I'd think that renaming the >> section would certainly be an option (though I'm unsure whether that would >> have other consequences - Rusty?), however I could also imagine other means >> to communicate to user land the width of a CRC value (e.g. adding an >> absolute symbol during the .ko linking stage). > >No, just break it once. And I still like the idea that we should do something >more radical if we're going to break this anyway, rather than these nasty asm >hacks. Actually I meanwhile think that module-init-tools can easily detect the changed layout without any further kernel side adjustments: Since it is known that a CRC always is a 32-bit value, simply checking whether the so-far-used 64-bit value has more than 32 significant bits should suffice: If so, the new layout is being used (with the symbol name starting at offset 4), else the old one is in effect (name at offset 8). This ought to be a pretty trivial change to that code. Jan -- 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/