netdev - Re: [PATCH net v2] virtio-net: don't disable guest csum when disable LRO

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Date:   Tue, 29 Sep 2020 09:31:23 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net v2] virtio-net: don't disable guest csum when disable LRO

On Tue, Sep 29, 2020 at 4:00 AM <xiangxia.m.yue@...il.com> wrote:
>
> From: Tonghao Zhang <xiangxia.m.yue@...il.com>
>
> Open vSwitch and Linux bridge will disable LRO of the interface
> when this interface added to them. Now when disable the LRO, the
> virtio-net csum is disable too. That drops the forwarding performance.

I had focused on the code previously.

The s/w checksum verification cost is significant in a VM with traffic
to local destinations. A bridge does not verify transport layer
checksums OTOH?

> Fixes: a02e8964eaf9 ("virtio-net: ethtool configurable LRO")
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Cc: Jason Wang <jasowang@...hat.com>
> Cc: Willem de Bruijn <willemb@...gle.com>
> Signed-off-by: Tonghao Zhang <xiangxia.m.yue@...il.com>
> ---
> v2:
> * change the fix-tag
> ---
>  drivers/net/virtio_net.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7145c83c6c8c..21b71148c532 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -63,6 +63,11 @@ static const unsigned long guest_offloads[] = {
>         VIRTIO_NET_F_GUEST_CSUM
>  };
>
> +#define GUEST_OFFLOAD_LRO_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))
> +
>  struct virtnet_stat_desc {
>         char desc[ETH_GSTRING_LEN];
>         size_t offset;
> @@ -2531,7 +2536,8 @@ static int virtnet_set_features(struct net_device *dev,
>                 if (features & NETIF_F_LRO)
>                         offloads = vi->guest_offloads_capable;
>                 else
> -                       offloads = 0;
> +                       offloads = vi->guest_offloads_capable &
> +                                  ~GUEST_OFFLOAD_LRO_MASK;
>
>                 err = virtnet_set_guest_offloads(vi, offloads);
>                 if (err)
> --
> 2.23.0
>