Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758290Ab2HUSIQ (ORCPT ); Tue, 21 Aug 2012 14:08:16 -0400 Received: from avon.wwwdotorg.org ([70.85.31.133]:59439 "EHLO avon.wwwdotorg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754161Ab2HUSIN (ORCPT ); Tue, 21 Aug 2012 14:08:13 -0400 Message-ID: <5033CE8A.5090705@wwwdotorg.org> Date: Tue, 21 Aug 2012 12:08:10 -0600 From: Stephen Warren User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120714 Thunderbird/14.0 MIME-Version: 1.0 To: Will Drewry CC: Tejun Heo , "linux-kernel@vger.kernel.org" , "cross-distro@lists.linaro.org" , Jens Axboe , Kay Sievers Subject: Re: root=PARTUUID for MBR/NT disk signatures? References: <502EC16C.4030007@wwwdotorg.org> <20120820182254.GQ24861@google.com> <50328258.9030800@wwwdotorg.org> In-Reply-To: X-Enigmail-Version: 1.5a1pre 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: 2933 Lines: 63 On 08/20/2012 10:47 PM, Will Drewry wrote: > On Mon, Aug 20, 2012 at 1:30 PM, Stephen Warren wrote: >> On 08/20/2012 12:22 PM, Tejun Heo wrote: >>> Hello, >>> >>> On Fri, Aug 17, 2012 at 04:10:52PM -0600, Stephen Warren wrote: >>>> I was considering extending the kernel command-line option >>>> root=PARTUUID= to also support MBR (NT disk signatures). I was thinking >>>> of a syntax along the lines of: >>>> >>>> root=PARTUUID=UUUUUUUU-PP[/PARTNROFF=%d] >>>> >>>> ... where UUUUUUUU is the hex representation of the NT disk signature, >>>> and PP is the hex representation of the partition number. Like GPT, >>>> /PARTNROFF could be used too if desired. >>>> >>>> Related, I was thinking of changing struct partition_meta_info's uuid >>>> field to be a string, so that it could simply be strcmp'd against the >>>> UUID value on the kernel command-line. That way, the type of the UUID is >>>> irrelevant. >>>> >>>> Does anyone have any objection to that? >>> >>> Wouldn't that be able to break setups which work currently? >> >> I don't believe so: >> >> Since the newly supported UUID syntax wouldn't ever match any EFI UUID >> (the lengths differ in all cases), I don't believe the new syntax would >> affect behavior for any existing usage. >> >> Obviously, part_efi.c would be modified to initialize struct >> partition_meta_info's uuid field to the appropriate string >> representation of the UUID so that the str(case)cmp would still succeed >> for existing command-lines. I ended up coding up that part of the change >> late Friday, and the feature was certainly still working OK. > > Functionally, I suspect this will work fine, but I am concerned that > it is a bad move from an efficiency perspective (not unfixable > though). Right now, the user-supplied value is converted from > string-uuid to packed-uuid. This is then memcmp'd across any and all > partitions - be it 2 or 200 - across all attached storage. If we move > to a pure string, then we end up needing to unpack every packed UUID > at disk scan time (or search, depending on impl) rather than just the > one user supplied value. The EFI partition code actually does the following already: 1) Unpack the UUID from the binary on-disk representation to a temporary string. 2) Repack the temporary string into the internal UUID buffer. The comments imply this is in order to do endian conversions. Switching the internal representation to a string avoids step (2) above, plus avoids having to pack the string on the kernel command-line into a binary UUID before the comparison. I doubt the difference between memcmp vs. strcasecmp is worth considering. So, I think it's overall a win. -- 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/