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Date: Fri, 2 Jun 2017 11:29:22 +0100
From: Roger Pau =?iso-8859-1?Q?Monn=E9?= <roger.pau@citrix.com>
To: Colin Percival <cperciva@FreeBSD.org>
CC: <src-committers@freebsd.org>, <svn-src-all@freebsd.org>,
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Subject: Re: svn commit: r319491 - head/sys/dev/xen/netfront
Message-ID: <20170602102922.stlu47ceumggu6o7@dhcp-3-128.uk.xensource.com>
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On Fri, Jun 02, 2017 at 07:03:31AM +0000, Colin Percival wrote:
> Author: cperciva
> Date: Fri Jun  2 07:03:31 2017
> New Revision: 319491
> URL: https://svnweb.freebsd.org/changeset/base/319491
> 
> Log:
>   Skip setting the MTU in the netfront driver (xn# devices) if the new MTU
>   is the same as the old MTU.  In particular, on Amazon EC2 "T2" instances
>   without this change, the network interface is reinitialized every 30
>   minutes due to the MTU being (re)set when a new DHCP lease is obtained,
>   causing packets to be dropped, along with annoying syslog messages about
>   the link state changing.
>   
>   As a side note, the behaviour this commit fixes was responsible for
>   exposing the locking problems fixed via r318523 and r318631.
>   
>   Maintainers of other network interface drivers may wish to consider making
>   the corresponding change; the handling of SIOCSIFMTU does not seem to
>   exhibit a great deal of consistency between drivers.

Is there any reason this check (ifp->if_mtu == ifr->ifr_mtu) is not
done at a higher level for all the drivers? It seems pointless to add
this chunk everywhere.

Roger.