Date:   Wed, 12 May 2021 15:53:17 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Yuri Benditovich <yuri.benditovich@...nix.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        linux-kernel <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 Wed, May 12, 2021 at 2:56 PM Yuri Benditovich
<yuri.benditovich@...nix.com> wrote:
>
> On Wed, May 12, 2021 at 5:33 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > On Wed, May 12, 2021 at 2:10 AM Yuri Benditovich
> > <yuri.benditovich@...nix.com> wrote:
> > >
> > > On Tue, May 11, 2021 at 8:48 PM Willem de Bruijn
> > > <willemdebruijn.kernel@...il.com> wrote:
> > > >
> > > > On Tue, May 11, 2021 at 12:43 AM Yuri Benditovich
> > > > <yuri.benditovich@...nix.com> wrote:
> > > > >
> > > > > 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;
> > > >
> > > > If adding a new VIRTIO_NET_HDR type I suggest adding separate IPv4 and
> > > > IPv6 variants, analogous to VIRTIO_NET_HDR_GSO_TCPV[46]. To avoid
> > > > having to infer protocol again, as for UDP fragmentation offload (the
> > > > retry case below this code).
> > >
> > > Thank you for denoting this important point of distinguishing between v4 and v6.
> > > Let's try to take a deeper look to see what is the correct thing to do
> > > and please correct me if I'm wrong:
> > > 1. For USO we do not need to guess the protocol as it is used with
> > > VIRTIO_NET_HDR_F_NEEDS_CSUM (unlike UFO)
> >
> > Enforcing that is a good start. We should also enforce that
> > skb->protocol is initialized to one of htons(ETH_P_IP) or
> > htons(ETH_P_IPV6), so that it does not have to be inferred by parsing.
>
> As this feature is new and is not used in any public release of any
> misbehaving driver, probably it is enough to state in the spec that
> VIRTIO_NET_HDR_F_NEEDS_CSUM is required for USO packets.
> The spec states that the USO feature requires checksumming feature.

The spec is not sufficient. These rules need to be enforced in the
kernel code, too.

> >
> > These requirements were not enforced for previous values, and cannot
> > be introduced afterwards, which has led to have to add that extra code
> > to handle these obscure edge cases.
> >
> > I agree that with well behaved configurations, the need for separate
> > _V4 and _V6 variants is not needed.
> >
> > > and the USO packets
> > > transmitted by the guest are under the same clause as both
> > > VIRTIO_NET_HDR_GSO_TCP, i.e. under if (hdr->flags &
> > > VIRTIO_NET_HDR_F_NEEDS_CSUM) {
> > > 2. If we even define VIRTIO_NET_HDR_GSO_UDPv4_L4 and
> > > VIRTIO_NET_HDR_GSO_UDPv6_L4 - both will be translated to
> > > SKB_GSO_UDP_L4, so this information is immediately lost (the code will
> > > look like:
> > > case VIRTIO_NET_HDR_GSO_UDP4_L4: case VIRTIO_NET_HDR_GSO_UDP6_L4
> > >     gso_type = SKB_GSO_UDP;
> > >
> > > 3. When we will define the respective guest features (like
> > > VIRTIO_NET_F_HOST_USO4 VIRTIO_NET_F_HOST_USO6) we will need to
> This is my typo: VIRTIO_NET_F_GUEST_USO4...
> > > recreate the virtio_net header from the skb when both v4 and v6 have
> > > the same SKB_GSO_UDP_L4, (see virtio_net_hdr_from_skb) and I'm not
> > > sure whether somebody needs the exact v4 or v6 information on guest RX
> > > path.
> >
> > FWIW, it is good to keep in mind that virtio_net_hdr is also used
> > outside virtio, in both ingress and egress paths.
>
> Can you please elaborate in which scenarios we do not have any virtio
> device in path but need virtio_net_hdr?

Packet sockets, tuntap.

> > > 4. What is completely correct is that when we will start working with
> > > the guest RX path we will need to define something like NETIF_F_USO4
> > > and NETIF_F_USO6 and configure them according to exact guest offload
> > > capabilities.
> > > Do you agree?
> >
> > I don't immediately see the need for advertising this device feature
> > on a per-protocol basis. Can you elaborate?
>
> Separate offload setting (controlled by the guest) for v4 and v6 in
> guest RX path is mandatory, at least Windows always requires this for
> any offload.
> In this case it seems easy to have also virtio-net device features to
> be indicated separately (the TAP/TUN should report its capabilities).

Ah, ok.

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