Hi,

I've been testing linux-next (tags later than 03/17) and hit new issues in
virtio-net on s390x. I bisected the issue, and I found this patch to be the
first buggy commit.

The issue seems to only be reproducible when running in Secure Execution.
Tested in a KVM guest, the virtio-net performance appears greatly reduced,
and the dmesg output shows many instances of the following error messages.

Partial relevant logs
=====================
[   49.332028] macvtap0: bad gso: type: 0, size: 0, flags 1 tunnel 0 tnl csum 0
[   74.365668] macvtap0: bad gso: type: 2e, size: 27948, flags 0 tunnel 0 tnl csum 0
[  403.302168] macvtap0: bad csum: flags: 2, gso_type: 23 rx_tnl_csum 0
[  403.302271] macvtap0: bad csum: flags: 2, gso_type: e0 rx_tnl_csum 0
[  403.302279] macvtap0: bad csum: flags: 2, gso_type: e1 rx_tnl_csum 0
[  403.309492] macvtap0: bad csum: flags: 2, gso_type: 4c rx_tnl_csum 0
[  403.317029] macvtap0: bad csum: flags: 2, gso_type: e0 rx_tnl_csum 0

Steps to reproduce
==================
1. Boot a Linux guest implementing this patch under QEMU/KVM (*) with SE
   enabled and a virtio-net-ccw device attached.
2. Run dmesg. The error message is usually already present at boot time,
   but if not, it can be reproduced by creating any network traffic.

(*) This patch was not tested in a non-KVM hypervisor environment.

I've further confirmed that reverting this patch onto its parent commit
resolves the issue. Please let me know if you'd like me to test a fix or if
you would need more information.

Thanks in advance.

Best,
Omar