Subject: Re: [net-next PATCH v5 1/6] net: virtio dynamically disable/enable LRO

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 9 Dec 2016 05:05:05 +0200

On Thu, Dec 08, 2016 at 04:04:58PM -0800, John Fastabend wrote:
> On 16-12-08 01:36 PM, Michael S. Tsirkin wrote:
> > On Wed, Dec 07, 2016 at 12:11:11PM -0800, John Fastabend wrote:
> >> This adds support for dynamically setting the LRO feature flag. The
> >> message to control guest features in the backend uses the
> >> CTRL_GUEST_OFFLOADS msg type.
> >>
> >> Signed-off-by: John Fastabend <[email-masked]>
> >> ---
> >>  drivers/net/virtio_net.c |   40 +++++++++++++++++++++++++++++++++++++++-
> >>  1 file changed, 39 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >> index a21d93a..a5c47b1 100644
> >> --- a/drivers/net/virtio_net.c
> >> +++ b/drivers/net/virtio_net.c
> >> @@ -1419,6 +1419,36 @@ static void virtnet_init_settings(struct net_device *dev)
> >>  	.set_settings = virtnet_set_settings,
> >>  };
> >>  
> >> +static int virtnet_set_features(struct net_device *netdev,
> >> +				netdev_features_t features)
> >> +{
> >> +	struct virtnet_info *vi = netdev_priv(netdev);
> >> +	struct virtio_device *vdev = vi->vdev;
> >> +	struct scatterlist sg;
> >> +	u64 offloads = 0;
> >> +
> >> +	if (features & NETIF_F_LRO)
> >> +		offloads |= (1 << VIRTIO_NET_F_GUEST_TSO4) |
> >> +			    (1 << VIRTIO_NET_F_GUEST_TSO6);
> >> +
> >> +	if (features & NETIF_F_RXCSUM)
> >> +		offloads |= (1 << VIRTIO_NET_F_GUEST_CSUM);
> >> +
> >> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS)) {
> >> +		sg_init_one(&sg, &offloads, sizeof(uint64_t));
> >> +		if (!virtnet_send_command(vi,
> >> +					  VIRTIO_NET_CTRL_GUEST_OFFLOADS,
> >> +					  VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET,
> >> +					  &sg)) {
> > 
> > Hmm I just realised that this will slow down setups that bridge
> > virtio net interfaces since bridge calls this if provided.
> > See below.
> 
> 
> Really? What code is trying to turn off GRO via the GUEST_OFFLOADS LRO
> command. My qemu/Linux setup has a set of tap/vhost devices attached to
> a bridge and all of them have LRO enabled even with this patch series.
> 
> I must missing a setup handler somewhere?

Turning off LRO, not GRO.

> > 
> >> +			dev_warn(&netdev->dev,
> >> +				 "Failed to set guest offloads by virtnet command.\n");
> >> +			return -EINVAL;
> >> +		}
> >> +	}
> > 
> > Hmm if VIRTIO_NET_F_CTRL_GUEST_OFFLOADS is off, this fails
> > silently. It might actually be a good idea to avoid
> > breaking setups.
> > 
> >> +
> >> +	return 0;
> >> +}
> >> +
> >>  static const struct net_device_ops virtnet_netdev = {
> >>  	.ndo_open            = virtnet_open,
> >>  	.ndo_stop   	     = virtnet_close,
> >> @@ -1435,6 +1465,7 @@ static void virtnet_init_settings(struct net_device *dev)
> >>  #ifdef CONFIG_NET_RX_BUSY_POLL
> >>  	.ndo_busy_poll		= virtnet_busy_poll,
> >>  #endif
> >> +	.ndo_set_features	= virtnet_set_features,
> >>  };
> >>  
> >>  static void virtnet_config_changed_work(struct work_struct *work)
> >> @@ -1815,6 +1846,12 @@ static int virtnet_probe(struct virtio_device *vdev)
> >>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM))
> >>  		dev->features |= NETIF_F_RXCSUM;
> >>  
> >> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) &&
> >> +	    virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6)) {
> >> +		dev->features |= NETIF_F_LRO;
> >> +		dev->hw_features |= NETIF_F_LRO;
> > 
> > So the issue is I think that the virtio "LRO" isn't really
> > LRO, it's typically just GRO forwarded to guests.
> > So these are easily re-split along MTU boundaries,
> > which makes it ok to forward these across bridges.
> > 
> > It's not nice that we don't document this in the spec,
> > but it's the reality and people rely on this.
> > 
> > For now, how about doing a custom thing and just disable/enable
> > it as XDP is attached/detached?
> 
> The annoying part about doing this is ethtool will say that it is fixed
> yet it will be changed by seemingly unrelated operation.

ATM ethtool does not tell you LRO is enabled, since
again, this isn't exactly LRO. It's a safe variant that
supports e.g. bridging.

> I'm not sure I
> like the idea to start automatically configuring the link via xdp_set.

That's what I thought up off-hand. Let me think about better
interfaces over the weekend.

> > 
> >> +	}
> >> +
> >>  	dev->vlan_features = dev->features;
> >>  
> >>  	/* MTU range: 68 - 65535 */
> >> @@ -2057,7 +2094,8 @@ static int virtnet_restore(struct virtio_device *vdev)
> >>  	VIRTIO_NET_F_CTRL_RX, VIRTIO_NET_F_CTRL_VLAN, \
> >>  	VIRTIO_NET_F_GUEST_ANNOUNCE, VIRTIO_NET_F_MQ, \
> >>  	VIRTIO_NET_F_CTRL_MAC_ADDR, \
> >> -	VIRTIO_NET_F_MTU
> >> +	VIRTIO_NET_F_MTU, \
> >> +	VIRTIO_NET_F_CTRL_GUEST_OFFLOADS
> >>  
> >>  static unsigned int features[] = {
> >>  	VIRTNET_FEATURES,