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

From: Tonghao Zhang <xiangxia.m.yue@gmail.com>
To: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Jason Wang <jasowang@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	virtualization@lists.linux-foundation.org,
	Network Development <netdev@vger.kernel.org>
Subject: Re: [PATCH net v2] virtio-net: don't disable guest csum when disable LRO
Date: Tue, 29 Sep 2020 15:52:50 +0800	[thread overview]
Message-ID: <CAMDZJNW=hEEcsJy1gUEwrnERRgH3kRBkEuDtcPwPdfXr91eTGg@mail.gmail.com> (raw)
In-Reply-To: <CA+FuTSebRQ=2VfT0KnM6ChjMg0j3NWJDPwn9S=aQk8tbNrUt6w@mail.gmail.com>

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). 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


> > Fixes: a02e8964eaf9 ("virtio-net: ethtool configurable LRO")
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Cc: Jason Wang <jasowang@redhat.com>
> > Cc: Willem de Bruijn <willemb@google.com>
> > Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.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
> >


-- 
Best regards, Tonghao