From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH -next] tun: stop tx queue when limit is hit
Date: Sun, 20 Jul 2014 22:33:13 -0700 (PDT)
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From: Florian Westphal <fw@strlen.de>
Date: Sun, 20 Jul 2014 20:51:25 +0200

> Currently tun just frees the skb and returns NETDEV_TX_OK
> when queue length exceeds txqlen.
> 
> This causes severe packetloss and unneeded resource
> consumption on host when sending to vm connected via tun.
> 
> Instead, lets stop the transmit queue and start it once
> packets are consumed from the queue.  This allows the network
> stack to control applications that send data via tun device.

I strongly suspect the current behavior is intentional, see
commit:

commit 5d097109257c03a71845729f8db6b5770c4bbedc
Author: Michael S. Tsirkin <mst@redhat.com>
Date:   Mon Dec 3 10:07:14 2012 +0000

    tun: only queue packets on device

for example.