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[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

** Also affects: linux (Ubuntu Xenial)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1781413

Title:
  Cannot set MTU higher than 1500 in Xen instance

Status in linux package in Ubuntu:
  New
Status in linux source package in Xenial:
  New

Bug description:
  [Impact]
  The latest Xenial update has broken MTU functionality in Xen: specifically, 
  setting MTUs larger than 1500 fails. This prevents Jumbo Frames and other 
  features which require larger than 1500 byte MTUs from being used. This can 
  lead to a failure to sync/connect to other components in the cluster/cloud
  which expect higher MTUs and result in unavailable services. 

  This can be worked around by manually using ethtool to set SCATTER/GATHER
  functionality:
  - $ sudo ethtool -K $interface_name sg on 

  
  The issue is caused by the following commit to the xen-netfront driver:
  "xen-netfront: Fix race between device setup and open"
  commit f599c64fdf7d9c108e8717fb04bc41c680120da4
  Introduced: v4.16-rc1

  Reverting the above fix has confirmed that the problem goes away.

  The following commits fix this issue in the mainline kernel:

  "xen-netfront: Fix mismatched rtnl_unlock"
  commit cb257783c2927b73614b20f915a91ff78aa6f3e8
  Introduced: v4.18-rc3
  "xen-netfront: Update features after registering netdev"
  commit 45c8184c1bed1ca8a7f02918552063a00b909bf5
  Introduced: v4.18-rc3

  
  [Test Case]
  1. Launch a Xen instance using the latest kernel version (e.g. 4.4.0-130,
  or  4.4.0-1062-aws)
  2. Change MTU to 9000 or other value > 1500. 

  [Regression Potential]
  The kernel patch might not be able to set MTU.

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