netdev - Re: [PATCH net-next] virtio-net: ethtool configurable LRO

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Date:   Wed, 19 Dec 2018 18:13:04 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jason Wang <jasowang@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] virtio-net: ethtool configurable LRO

On Wed, Dec 19, 2018 at 05:44:44PM -0500, Willem de Bruijn wrote:
> On Wed, Dec 19, 2018 at 5:32 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > Virtio-net devices negotiate LRO support with the host.
> > Display the initially negotiated state with ethtool -k.
> >
> > Also allow configuring it with ethtool -K, reusing the existing
> > virtnet_set_guest_offloads helper that disables LRO for XDP.
> >
> > Virtio-net negotiates TSO4 and TSO6 separately, but ethtool does not
> > distinguish between the two. In the unlikely legacy case where the
> > host only supports one of the two, maintain the current state.
> >
> > RTNL is held while calling virtnet_set_features, same as on the path
> > from virtnet_xdp_set.
> >
> > Tested:
> >   Verified that large-receive-offload is "off [fixed]" when not
> >   negotiated during initial probe and "on" otherwise.
> >
> >   Measured packet size when scp-ing a large file to the guest with
> >   nstat (IpExtInOctets / IpInDelivers). Verified that packet size
> >   exceeds MTU only if ethtool -k shows LRO and/or GRO as enabled.
> >
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> 
> Two comments for the review:
> 
> 1. Current drivers show LRO as disabled even if it is negotiated at
> probe. That is arguably a bug. In which case I can split this patch into
> - a net patch to advertise the initially negotiated value through dev->feature
> - a net-next patch to configure it with ndo_set_features.
> 
> 2. virtnet_clear_guest_offloads disables all guest_offloads, including
> VIRTIO_NET_F_GUEST_CSUM. That seems undesirable? The feature is a
> prerequisite for LRO, but not the other way around. This applies
> equally to the xdp and ethtool paths, so would be another  separate
> net patch.

Makes sense I guess.