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Date:   Sun, 12 Mar 2017 19:26:27 -0700 (PDT)
From:   David Miller <>
Subject: Re: tun offloads bug

From: Yaroslav Isakov <>
Date: Wed, 8 Mar 2017 23:29:53 +0300

> Hello!
> I've found a bug in TUN/TAP driver with offloads - when Qemu is trying
> to set offloads on tap device, there is no error, but offloads are not
> appied.
> The cause of this is that udev in recent systemd is using ethtool to
> disable offloads. So, udev is setting tun->dev->wanted_features via
> ethtool ioctl to disable TSO, but when qemu is trying to set offloads,
> it's using TUN/TAP ioctl which is not setting wanted_features, so
> netdev_update_features will not see features qemu wants.
> This can be easily reproduced - just run qemu with
> guest_tso4=on,guest_tso6=on on systemd with systemd>=226 and after
> booting the VM, ethtool -k tap0 will show that TSO4 is disabled and
> TSO6 is enabled (systemd is not touching TSO6, that's why it's not
> affected at all)
> I've attached pretty trivial patch to fix this problem.

Please read Documentation/SubmittingPatches for the proper way to
submit a patch.

In particular you have to provide a proper signoff tag.

Thank you.

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