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

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Tue, Sep 29, 2020 at 9:56 AM Tonghao Zhang <xiangxia.m.yue@gmail.com> wrote:
>
> On Tue, Sep 29, 2020 at 3:32 PM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > On Tue, Sep 29, 2020 at 4:00 AM <xiangxia.m.yue@gmail.com> wrote:
> > >
> > > From: Tonghao Zhang <xiangxia.m.yue@gmail.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?
> Hi Willem.
> No, think about GRO(In the GRO we don't know packets will be forwarded
> to other ports or to local).

I had expected a pure bridging setup that disables LRO to disable GRO as well.

But if not, then, indeed, the checksum needs to be verified before
coalescing. Makes sense.

> The call tree as below:
>    + 5.41% secondary_startup_64
>    - 1.22% ret_from_fork
> ....
>         net_rx_action
>         napi_poll
>         virtnet_poll
>         virtnet_receive
>         napi_gro_receive
>         dev_gro_receive
>         inet_gro_receive
>         tcp4_gro_receive
>         __skb_gro_checksum_complete
>         skb_checksum
>         __skb_checksum
>         csum_partial
>         do_csum
>    - 1.13% do_csum
>
> $ brctl show
> bridge name bridge id STP enabled interfaces
> br0 8000.001122330001 no eth1
> eth2