linux-kernel - Re: [PATCH 1/3] block: sed-opal: add ioctl for done-mark of shadow mbr

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Date:   Wed, 1 May 2019 12:36:40 +0200 (CEST)
From:   David Kozub <zub@...ux.fjfi.cvut.cz>
To:     Jens Axboe <axboe@...nel.dk>,
        Jonathan Derrick <jonathan.derrick@...el.com>,
        Scott Bauer <sbauer@...donthack.me>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
cc:     Jonas Rabenstein <jonas.rabenstein@...dium.uni-erlangen.de>
Subject: Re: [PATCH 1/3] block: sed-opal: add ioctl for done-mark of shadow
 mbr

On Wed, 1 May 2019, David Kozub wrote:

> @@ -104,6 +109,12 @@ struct opal_mbr_data {
> 	__u8 __align[7];
> };
>
> +struct opal_mbr_done {
> +	struct opal_key key;
> +	__u8 done_flag;
> +	__u8 __align[7];
> +};

While I just copied opal_mbr_data here, I wonder what is the point of 
__align in these structs. By itself it just pads the structure to have a 
size that is a multiple of 8. Is this to make sure that anything that lies 
past the structure is 8-bytes aligned (assuming the start is 8-bytes 
aligned too), perhaps for 32bit userspace with 64bit kernel?

And if it's this, is it needed for these IOCTL structs? (I can see it 
being useful for struct opal_key.)

Best regards,
David