linux-kernel - Re: [PATCH v2 3/3] blk-crypto: show crypto capabilities in sysfs

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Date:   Tue, 30 Nov 2021 07:49:54 +0100
From:   Hannes Reinecke <hare@...e.de>
To:     Eric Biggers <ebiggers@...nel.org>, linux-block@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-mmc@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH v2 3/3] blk-crypto: show crypto capabilities in sysfs

On 11/30/21 5:03 AM, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> Add sysfs files that expose the inline encryption capabilities of
> request queues:
> 
> 	/sys/class/block/$disk/queue/crypto/max_dun_bits
> 	/sys/class/block/$disk/queue/crypto/modes/$mode
> 	/sys/class/block/$disk/queue/crypto/num_keyslots
> 
> Userspace can use these new files to decide what encryption settings to
> use, or whether to use inline encryption at all.  This also brings the
> crypto capabilities in line with the other queue properties, which are
> already discoverable via the queue directory in sysfs.
> 
> Design notes:
> 
>    - Place the new files in a new subdirectory "crypto" to group them
>      together and to avoid complicating the main "queue" directory.  This
>      also makes it possible to replace "crypto" with a symlink later if
>      we ever make the blk_crypto_profiles into real kobjects (see below).
> 
>    - It was necessary to define a new kobject that corresponds to the
>      crypto subdirectory.  For now, this kobject just contains a pointer
>      to the blk_crypto_profile.  Note that multiple queues (and hence
>      multiple such kobjects) may refer to the same blk_crypto_profile.
> 
>      An alternative design would more closely match the current kernel
>      data structures: the blk_crypto_profile could be a kobject itself,
>      located directly under the host controller device's kobject, while
>      /sys/class/block/$disk/queue/crypto would be a symlink to it.
> 
>      I decided not to do that for now because it would require a lot more
>      changes, such as no longer embedding blk_crypto_profile in other
>      structures, and also because I'm not sure we can rule out moving the
>      crypto capabilities into 'struct queue_limits' in the future.  (Even
>      if multiple queues share the same crypto engine, maybe the supported
>      data unit sizes could differ due to other queue properties.)  It
>      would also still be possible to switch to that design later without
>      breaking userspace, by replacing the directory with a symlink.
> 
>    - Use "max_dun_bits" instead of "max_dun_bytes".  Currently, the
>      kernel internally stores this value in bytes, but that's an
>      implementation detail.  It probably makes more sense to talk about
>      this value in bits, and choosing bits is more future-proof.
> 
>    - "modes" is a sub-subdirectory, since there may be multiple supported
>      crypto modes, and sysfs is supposed to have one value per file.
> 
Why do you have a sub-directory here?
 From what I can see, that subdirectory just contains the supported 
modes, so wouldn't it be easier to create individual files like 
'mode_<modename>' instead of a subdirectory?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@...e.de                              +49 911 74053 688
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