netdev - Re: [PATCH 2/4] virtio-net: add support of UDP segmentation (USO) on the host

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Date:   Tue, 11 May 2021 16:31:10 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yuri Benditovich <yuri.benditovich@...nix.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        Yan Vugenfirer <yan@...nix.com>
Subject: Re: [PATCH 2/4] virtio-net: add support of UDP segmentation (USO) on
 the host

On Tue, May 11, 2021 at 4:24 PM Yuri Benditovich
<yuri.benditovich@...nix.com> wrote:
>
> On Tue, May 11, 2021 at 9:47 AM Jason Wang <jasowang@...hat.com> wrote:
> >
> >
> > 在 2021/5/11 下午12:42, Yuri Benditovich 写道:
> > > Large UDP packet provided by the guest with GSO type set to
> > > VIRTIO_NET_HDR_GSO_UDP_L4 will be divided to several UDP
> > > packets according to the gso_size field.
> > >
> > > Signed-off-by: Yuri Benditovich <yuri.benditovich@...nix.com>
> > > ---
> > >   include/linux/virtio_net.h | 5 +++++
> > >   1 file changed, 5 insertions(+)
> > >
> > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > > index b465f8f3e554..4ecf9a1ca912 100644
> > > --- a/include/linux/virtio_net.h
> > > +++ b/include/linux/virtio_net.h
> > > @@ -51,6 +51,11 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
> > >                       ip_proto = IPPROTO_UDP;
> > >                       thlen = sizeof(struct udphdr);
> > >                       break;
> > > +             case VIRTIO_NET_HDR_GSO_UDP_L4:
> > > +                     gso_type = SKB_GSO_UDP_L4;
> > > +                     ip_proto = IPPROTO_UDP;
> > > +                     thlen = sizeof(struct udphdr);
> > > +                     break;
> >
> >
> > This is only for rx, how about tx?
>
> In terms of the guest this is only for TX.

So virtio_net_hdr_to_skb() can be called by all the followings:

1) receive_buf() which is guest RX.
2) tun_get_user() which is guest TX
3) tap_get_user() which is guest TX
4) {t}packet_send() which is userspace TX

So it touches for both RX and TX.

> Guest RX is a different thing, this is actually coalescing of
> segmented UDP packets into a large one.

Another case, the packet could be sent from another VM (like the UFO case).

Supporting that for both TX and RX and greatly improve the performance
of VM2VM traffic.

Thanks

> This feature is not defined in the virtio spec yet and the support of
> it first of all depends on the OS.
> For example: TCP LSO (guest TX) is supported almost by all the
> versions of Windows.
> TCP RSC (coalescing of TCP segments) is supported by Win 8 / Server 2012 and up.
> UDP segmentation is supported by Windows kernels 1903+
> UDP coalescing is defined by Windows kernels 2004+ and not supported
> by the driver yet.
>
> >
> > Thanks
> >
> >
> >
> > >               default:
> > >                       return -EINVAL;
> > >               }
> >
>