Re: root=PARTUUID for MBR/NT disk signatures?

[Stephen Warren <swarren@wwwdotorg.org>]

On 08/20/2012 10:47 PM, Will Drewry wrote:
> On Mon, Aug 20, 2012 at 1:30 PM, Stephen Warren <swarren@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.