Re: [PATCH] qapi: Craft the BlockdevCreateOptionsLUKS comment

[Markus Armbruster <armbru@redhat.com>]

Yong Huang <yong.huang@smartx.com> writes:

> On Wed, Feb 21, 2024 at 2:43 PM Markus Armbruster <armbru@redhat.com> wrote:
>
>> Hyman Huang <yong.huang@smartx.com> writes:
>>
>> > Add comment in detail for commit 433957bb7f (qapi:
>> > Make parameter 'file' optional for
>> > BlockdevCreateOptionsLUKS).
>> >
>> > Signed-off-by: Hyman Huang <yong.huang@smartx.com>
>> > ---
>> >  qapi/block-core.json | 20 +++++++++++++++++++-
>> >  1 file changed, 19 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/qapi/block-core.json b/qapi/block-core.json
>> > index ab5a93a966..42b0840d43 100644
>> > --- a/qapi/block-core.json
>> > +++ b/qapi/block-core.json
>> > @@ -4973,7 +4973,25 @@
>> >  ##
>> >  # @BlockdevCreateOptionsLUKS:
>> >  #
>> > -# Driver specific image creation options for LUKS.
>> > +# Driver specific image creation options for LUKS. Note that
>> > +# @file is required if @preallocation is specified and equals
>> > +# PREALLOC_MODE_ON. The following three scenarios determine how
>> > +# creation logic behaves when @preallocation is either equal to
>> > +# PREALLOC_MODE_OFF or is not given:
>> > +#
>> > +#  1) When @file is given only, format the block device referenced
>> > +#     by @file as the LUKS specification and trunk it to the @size.
>>
>> Do you mean "truncate it to @size"?
>>
> Yes, :( sorry for the spelling mistake.

Writing good documentation in a second language is *hard*.  All we can
reasonably expect from contributors to try their best.  And then we
improve the text together in review.  Just like we do for code :)

>> > +#     In this case, the @size should reflect amount of space made
>> > +#     available to the guest, so the trunk size must take account
>> > +#     of that which will be used by the crypto header.
>> > +#
>> > +#  2) When @header is given only, just format the block device
>> > +#     referenced by @header as the LUKS specification.
>> > +#
>> > +#  3) When both @file and @header are given, block device
>> > +#     referenced by @file should be trunked to @size, and block
>> > +#     device referenced by @header should be formatted as the LUKS
>> > +#     specification.
>> >  #
>> >  # @file: Node to create the image format on, mandatory except when
>> >  #        'preallocation' is not requested
>>
>> Let's see whether I understand.
>>
>> blockdev-create with "driver": "luks" can work in three different ways:
>>
>> 1. Create an image with a LUKS header
>>
>> 2. Create just a detached LUKS header
>>
>> 3. Create an image and a detached LUKS header
>>
>> Correct?
>>
>
> Yes
>
>
>> @file and @header are BlockdevRef, which means they refer to existing
>> images with arbitrary driver.  Could be "file", "qcow2", or anything.
>>
>> Correct?
>>
> Yes
>
>
>>
>> To get 1., specify @file, but not @header.
>>
>> To get 2., specify @header, but not @file.
>>
>> To get 3., specify both.
>>
>> Specifying neither is an error.
>>
>> Correct?
>>
>
> Yes
>
>
>> In any case, @size is the logical size of the image (how much data it
>> can hold).
>>
>
> Yes
>
>
>>
>> With 1., the actual image size is a bit larger due to the LUKS header.
>> The @file image is resized to that size: if it's shorter, it's grown, if
>> it's longer, it's truncated.
>>
>
> Yes
>
>
>> With 2., @size is merely recorded in the detached LUKS header.
>>
>
> In LUKS1 specification, payload data size is not contained in the header,
> so in this case, @size is not recorded in the detached LUKS header.
> The creation logic just does the LUKS header formatting only.

Is @size unused then?

>> With 3., @size is recorded in the detached LUKS header, and the @file
>> image is resized as with 1.
>>
>
> Same reason as above, @size is not recorded and @file image is
> resized to @size exactly.
>
>
>>
>> Correct?
>>
>>
> Thanks for the comment,
>
> Yong