linux-kernel - Re: [PATCH v2] s390/vfio-ap: Driver feature advertisement

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Message-ID: <20240911065707.6563-A-hca@linux.ibm.com>
Date: Wed, 11 Sep 2024 08:57:07 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: "Jason J. Herne" <jjherne@...ux.ibm.com>
Cc: linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        fiuczy@...ux.ibm.com, akrowiak@...ux.ibm.com, borntraeger@...ibm.com,
        agordeev@...ux.ibm.com, gor@...ux.ibm.com
Subject: Re: [PATCH v2] s390/vfio-ap: Driver feature advertisement

On Tue, Sep 10, 2024 at 07:34:40AM -0400, Jason J. Herne wrote:
> Advertise features of the driver for the benefit of automated tooling
> like Libvirt and mdevctl.
> 
> Signed-off-by: Jason J. Herne <jjherne@...ux.ibm.com>
> Reviewed-by: Anthony Krowiak <akrowiak@...ux.ibm.com>
> Reviewed-by: Boris Fiuczynski <fiuczy@...ux.ibm.com>
> ---
>  Documentation/arch/s390/vfio-ap.rst | 34 +++++++++++++++++++++++++++++
>  drivers/s390/crypto/vfio_ap_drv.c   | 13 +++++++++++
>  2 files changed, 47 insertions(+)

...

> +Driver Features
> +===============
> +The vfio_ap driver exposes a sysfs file containing supported features.
> +This exists so third party tools (like Libvirt and mdevctl) can query the
> +availability of specific features.
> +
> +The features list can be found here: /sys/bus/matrix/devices/matrix/features
> +
> +Entries are \n delimited. Each entry contains a key value pair. The key is made
> +up of a combination of alphanumeric and underscore characters. The separator
> +consists of a space, a colon and then another space. The value consists of
> +alphanumeric, space, and underscore characters.
> +
> +Example:
> +cat /sys/bus/matrix/devices/matrix/features
> +flags : guest_matrix dyn ap_config
> +
> +Presently only a single field named flags is defined. It is meant to advertise a
> +list of features the driver provides. The flags fields advertises the following
> +features:

I stumbled across this only now: sysfs files are not supposed to have
several key value pairs. Actually the file(name) itself is supposed to
be the key and its contents are the value. So I would expect:

cat /sys/bus/matrix/devices/matrix/flags
guest_matrix dyn ap_config

Which is also easier to parse. Is there a good reason why this does
not follow the general approach for sysfs files?