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

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Date:	Tue, 21 Aug 2012 13:47:02 -0500
From:	Will Drewry <wad@...omium.org>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"cross-distro@...ts.linaro.org" <cross-distro@...ts.linaro.org>,
	Jens Axboe <axboe@...nel.dk>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: root=PARTUUID for MBR/NT disk signatures?

On Tue, Aug 21, 2012 at 1:08 PM, Stephen Warren <swarren@...dotorg.org> wrote:
> On 08/20/2012 10:47 PM, Will Drewry wrote:
>> On Mon, Aug 20, 2012 at 1:30 PM, Stephen Warren <swarren@...dotorg.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
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