On Tue, Aug 21, 2012 at 1:08 PM, Stephen Warren <swar...@wwwdotorg.org> wrote: > 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.
Sounds reasonable to me then. Thanks! will -- 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/
