netdev - Re: tun offloads bug

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Date:   Thu, 16 Mar 2017 21:23:51 +0300
From:   Yaroslav Isakov <yaroslav.isakov@...il.com>
To:     netdev@...r.kernel.org
Subject: Re: tun offloads bug

Hello! I'm sending new patch, with proper formatting and signed tag

2017-03-08 23:29 GMT+03:00 Yaroslav Isakov <yaroslav.isakov@...il.com>:
> 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.

View attachment "0001-tun-fix-inability-to-set-offloads-after-disabling-th.patch" of type "text/x-patch" (982 bytes)