netdev - Re: [xdp-hints] [PATCH bpf-next V1] xdp: bpf_xdp_metadata use NODEV for no device support

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Message-ID: <c9be8991-1186-ef0f-449c-f2dd5046ca02@intel.com>
Date:   Wed, 15 Feb 2023 18:11:48 +0100
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
CC:     Larysa Zaremba <larysa.zaremba@...el.com>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, Stanislav Fomichev <sdf@...gle.com>,
        <martin.lau@...nel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
        <alexandr.lobakin@...el.com>, <xdp-hints@...-project.net>
Subject: Re: [xdp-hints] [PATCH bpf-next V1] xdp: bpf_xdp_metadata use NODEV
 for no device support

From: Zaremba, Larysa <larysa.zaremba@...el.com>
Date: Wed, 15 Feb 2023 16:45:18 +0100

> On Wed, Feb 15, 2023 at 11:09:36AM +0100, Jesper Dangaard Brouer wrote:
>> With our XDP-hints kfunc approach, where individual drivers overload the
>> default implementation, it can be hard for API users to determine
>> whether or not the current device driver have this kfunc available.
>>
>> Change the default implementations to use an errno (ENODEV), that
>> drivers shouldn't return, to make it possible for BPF runtime to
>> determine if bpf kfunc for xdp metadata isn't implemented by driver.
> 
> I think it diverts ENODEV usage from its original purpose too much. Maybe 
> providing information in dmesg would be a better solution?

+1, -%ENODEV shouldn't be used here. It stands for "no device", for
example the driver probing core doesn't treat it as an error or that
something is not supported (rather than there's no device installed
in a slot / on a bus etc.).

> 
>>
>> This is intended to ease supporting and troubleshooting setups. E.g.
>> when users on mailing list report -19 (ENODEV) as an error, then we can
>> immediately tell them their kernel is too old.
> 
> Do you mean driver being too old, not kernel?
> 
>>
>> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
>> ---
[...]

Thanks,
Olek