On 08/20/2012 10:47 PM, Will Drewry wrote: > On Mon, Aug 20, 2012 at 1:30 PM, Stephen Warren <swar...@wwwdotorg.org> 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 majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
