Re: [PATCH net] virtio-net: use NETIF_F_GRO_HW instead of NETIF_F_LRO

["Michael S. Tsirkin" <mst@redhat.com>]

Patch is good. Suggest some tweaks to the commit log.

On Tue, Aug 17, 2021 at 10:03:38AM +0800, Jason Wang wrote:
> Commit a02e8964eaf92 ("virtio-net: ethtool configurable LRO") tries to
> advertise LRO on behalf of the guest offloading features and allow the

tries to advertise -> advertises

that part actually works.

allow -> allows

on behalf of features is really weird. maybe "maps"?

> administrator to enable and disable those features via ethtool.
> 
> This may lead several issues:

may lead->lead to

> 
> - For the device that doesn't support control guest offloads, the
>   "LRO" can't be disabled so we will get a warn in the

warn -> warning

>   dev_disable_lro()

.. when turning off LRO or when enabling forwarding bridging etc.

> - For the device that have the control guest offloads, the guest

have the -> supports

>   offloads were disabled in the case of bridge etc

etc -> forwarding etc

> which may slow down

were -> are

may slow -> slows

>   the traffic.
> 
> Fixing this by using NETIF_F_GRO_HW instead. Though the spec does not
> guaranteed to be re-segmented as original explicitly now, we can add

guaranteed -> guarantee

> that to the spec

I would add:

Further, we never advertised LRO historically before a02e8964eaf92
("virtio-net: ethtool configurable LRO") and so bridged/forwarded
configs effectively relied on virtio receive offloads being GRO.




> and then we can catch the bad configuration and
> setup.

Don't know what does this part mean. How would we catch it?
With a new flag? Let's say so.

> 
> Fixes: a02e8964eaf92 ("virtio-net: ethtool configurable LRO")
> Signed-off-by: Jason Wang <jasowang@redhat.com>



Proposed rewritten commit log:

===
[PATCH net] virtio-net: use NETIF_F_GRO_HW instead of NETIF_F_LRO

Commit a02e8964eaf92 ("virtio-net: ethtool configurable LRO")
maps LRO to virtio guest offloading features and allows the
administrator to enable and disable those features via ethtool.
 
This leads to several issues:


- For a device that doesn't support control guest offloads, the "LRO"
  can't be disabled triggering WARN in dev_disable_lro() when turning
  off LRO or when enabling forwarding bridging etc.

- For a device that supports control guest offloads, the guest
  offloads are disabled in cases of bridging, forwarding etc
  slowing down the traffic.
 
Fix this by using NETIF_F_GRO_HW instead. Though the spec does not
guarantee packets to be re-segmented as the original ones,
we can add that to the spec, possibly with a flag for devices to
differentiate between GRO and LRO.

Further, we never advertised LRO historically before a02e8964eaf92
("virtio-net: ethtool configurable LRO") and so bridged/forwarded
configs effectively always relied on virtio receive offloads behaving
like GRO - thus even if this breaks any configs it is at least not
a regression.

Fixes: a02e8964eaf92 ("virtio-net: ethtool configurable LRO")
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reported-by: Ivan <ivan@prestigetransportation.com>
Tested-by: Ivan <ivan@prestigetransportation.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

===


> ---
>  drivers/net/virtio_net.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 0416a7e00914..10c382b08bce 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -63,7 +63,7 @@ static const unsigned long guest_offloads[] = {
>  	VIRTIO_NET_F_GUEST_CSUM
>  };
>  
> -#define GUEST_OFFLOAD_LRO_MASK ((1ULL << VIRTIO_NET_F_GUEST_TSO4) | \
> +#define GUEST_OFFLOAD_GRO_HW_MASK ((1ULL << VIRTIO_NET_F_GUEST_TSO4) | \
>  				(1ULL << VIRTIO_NET_F_GUEST_TSO6) | \
>  				(1ULL << VIRTIO_NET_F_GUEST_ECN)  | \
>  				(1ULL << VIRTIO_NET_F_GUEST_UFO))
> @@ -2481,7 +2481,7 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>  	        virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_ECN) ||
>  		virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_UFO) ||
>  		virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_CSUM))) {
> -		NL_SET_ERR_MSG_MOD(extack, "Can't set XDP while host is implementing LRO/CSUM, disable LRO/CSUM first");
> +		NL_SET_ERR_MSG_MOD(extack, "Can't set XDP while host is implementing GRO_HW/CSUM, disable GRO_HW/CSUM first");
>  		return -EOPNOTSUPP;
>  	}
>  
> @@ -2612,15 +2612,15 @@ static int virtnet_set_features(struct net_device *dev,
>  	u64 offloads;
>  	int err;
>  
> -	if ((dev->features ^ features) & NETIF_F_LRO) {
> +	if ((dev->features ^ features) & NETIF_F_GRO_HW) {
>  		if (vi->xdp_enabled)
>  			return -EBUSY;
>  
> -		if (features & NETIF_F_LRO)
> +		if (features & NETIF_F_GRO_HW)
>  			offloads = vi->guest_offloads_capable;
>  		else
>  			offloads = vi->guest_offloads_capable &
> -				   ~GUEST_OFFLOAD_LRO_MASK;
> +				   ~GUEST_OFFLOAD_GRO_HW_MASK;
>  
>  		err = virtnet_set_guest_offloads(vi, offloads);
>  		if (err)
> @@ -3100,9 +3100,9 @@ static int virtnet_probe(struct virtio_device *vdev)
>  		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->features |= NETIF_F_GRO_HW;
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS))
> -		dev->hw_features |= NETIF_F_LRO;
> +		dev->hw_features |= NETIF_F_GRO_HW;
>  
>  	dev->vlan_features = dev->features;
>  
> -- 
> 2.25.1

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