netdev - Re: [RFE net-next] net: tun: 1000x speed up

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Date:   Mon, 31 Oct 2022 18:09:58 +0100
From:   Ilya Maximets <i.maximets@....org>
To:     netdev@...r.kernel.org
Cc:     i.maximets@....org, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFE net-next] net: tun: 1000x speed up

On 10/21/22 13:49, Ilya Maximets wrote:
> The 10Mbps link speed was set in 2004 when the ethtool interface was
> initially added to the tun driver.  It might have been a good
> assumption 18 years ago, but CPUs and network stack came a long way
> since then.
> 
> Other virtual ports typically report much higher speeds.  For example,
> veth reports 10Gbps since its introduction in 2007.
> 
> Some userspace applications rely on the current link speed in
> certain situations.  For example, Open vSwitch is using link speed
> as an upper bound for QoS configuration if user didn't specify the
> maximum rate.  Advertised 10Mbps doesn't match reality in a modern
> world, so users have to always manually override the value with
> something more sensible to avoid configuration issues, e.g. limiting
> the traffic too much.  This also creates additional confusion among
> users.
> 
> Bump the advertised speed to at least match the veth.  10Gbps also
> seems like a more or less fair assumption these days, even though
> CPUs can do more.  Alternative might be to explicitly report UNKNOWN
> and let the application/user decide on a right value for them.
> 
> Link: https://mail.openvswitch.org/pipermail/ovs-discuss/2022-July/051958.html
> Signed-off-by: Ilya Maximets <i.maximets@....org>
> ---
> 
> Sorry for the clickbait subject line.  Can change it to something more
> sensible while posting non-RFE patch.  Something like:
> 
>   'net: tun: bump the link speed from 10Mbps to 10Gbps'
> 
> This patch is RFE just to start a conversation.

OK.  There seems to be no more discussions around the topic, so
I'll make a conclusion.

General understanding is that reporting UNKNOWN will cause problems
with bonding (not sure why anyone will add tap into bonding outside
of just for testing reasons, but that's a different topic) and will
potentially cause problems with userpsace applications that do not
handle that case for some reason.  So, this is a risky option at the
moment.

There was no strong opinion against equalizing speeds between veth
and tun/tap.  Sounds like a safe option in general and there are no
known use cases that will be negatively affected.

So, I think, I'll go ahead and post the non-RFC version of the
proposed change.

Thanks!

Best regards, Ilya Maximets.