netdev - Re: TUN/TAP: tap driver reports bogus interface speed in ethtool operations

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Date:	Tue, 23 Jul 2013 08:49:01 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Helmut Grohne <h.grohne@...nusnetworks.de>
CC:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
	Max Krasnyansky <maxk@....qualcomm.com>
Subject: Re: TUN/TAP: tap driver reports bogus interface speed in ethtool
 operations

On 07/23/2013 06:32 AM, Helmut Grohne wrote:
>
> On 16.07.2013, at 23:41, Max Krasnyansky <maxk@....qualcomm.com> wrote:
>> The patch looks fine to me (must admit that I only glanced at it). Please send it to the netdev
>> mailing list netdev@...r.kernel.org, if you have't already done so.
>> CC David Miller <davem@...emloft.net> and me.
>
> Doing that. Now summarizing the issue for the new recipients:
>
> Problem:
>
> When querying a tap device for its speed using ethtool the tun driver reports a speed
> of SPEED_10. This number is hard coded into the driver. Nowadays virtual network devices
> can go way faster than that. When doing automatic bandwidth monitoring based on the
> speed reported by ethtool, tap devices tend to come up as false positives. Arguably the
> hard coded speed is wrong.
>
> Proposed solution:
>
> To that end I propose supporting the ETHTOOL_SSET command in addition to the already
> supported ETHTOOL_GSET. It would deny setting any setting except for the bandwidth
> where it would allow arbitrary values. You can find this patch attached.
>
> With this patch an administrator can increase the reported speed for tap devices and
> keep using automatic detection of interface speeds.
>
> Workarounds:
>
> Using ethtool a detection utility can determine the driver for an interface. If the
> driver matches the string "tun", the reported speed should not be used.
>
> Helmut
>

I like the idea of being able to set the reported speed for a tun/tap 
interface.

rick jones
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