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

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Message-ID: <20130723085611.0c77c314@nehalam.linuxnetplumber.net>
Date:	Tue, 23 Jul 2013 08:56:11 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Rick Jones <rick.jones2@...com>
Cc:	Helmut Grohne <h.grohne@...nusnetworks.de>, 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 Tue, 23 Jul 2013 08:49:01 -0700
Rick Jones <rick.jones2@...com> wrote:

> 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

I have hack patch to allow overwriting speed, info, and statistics that is part
of how we use TUN as dummy devices. How far do you think makes sense?


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