netdev - Re: regression: UFO removal breaks kvm live migration

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Message-Id: <20171108.152624.282888162096095926.davem@davemloft.net>
Date:   Wed, 08 Nov 2017 15:26:24 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     willemdebruijn.kernel@...il.com
Cc:     mkubecek@...e.cz, netdev@...r.kernel.org
Subject: Re: regression: UFO removal breaks kvm live migration

From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date: Wed, 8 Nov 2017 12:36:26 +0900

> On Tue, Nov 7, 2017 at 5:02 PM, Michal Kubecek <mkubecek@...e.cz> wrote:
>> I didn't have time to think it through yet but perhaps we could allow
>> setting TUN_F_UFO and ignore its value.
> 
> If the feature is enabled guests may try to send UFO packets, which
> the host is no longer able to fragment.
> 
> virtio_net_hdr_to_skb will drop the packets immediately based on
> gso_type and tun_get_user will return EINVAL.
> 
> Still, perhaps that's preferable as migration will succeed and most
> guests won't ever try to send those packets in the first place.

However, this would create the situation where there is no way
to properly probe for the actual presence of UFO support.