netdev - network performance degradation in virtio

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Date:   Mon, 22 May 2017 10:25:19 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org
Subject: network performance degradation in virtio_net in 4.12-rc

Hi

I see severe network performance degradation with the kernels 4.12-rc1 and 
4.12-rc2 in the network virtio driver. Download rate drops down to about 
100kB/s.

I bisected it and it is caused by patch 
d85b758f72b05a774045545f24d70980e3e9aac4 ("virtio_net: fix support for 
small rings"). When I revert this patch, the problem goes away.

The host is Debian Jessie with kernel 4.4.62, the guest is Debian Sid with 
kernel 4.12-rc.

Mikulas