Re: [RFC PATCH 0/7] Support for virtio-net hash reporting

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

On Tue, Jan 5, 2021 at 8:12 AM Yuri Benditovich
<yuri.benditovich@daynix.com> wrote:
>
> Existing TUN module is able to use provided "steering eBPF" to
> calculate per-packet hash and derive the destination queue to
> place the packet to. The eBPF uses mapped configuration data
> containing a key for hash calculation and indirection table
> with array of queues' indices.
>
> This series of patches adds support for virtio-net hash reporting
> feature as defined in virtio specification. It extends the TUN module
> and the "steering eBPF" as follows:
>
> Extended steering eBPF calculates the hash value and hash type, keeps
> hash value in the skb->hash and returns index of destination virtqueue
> and the type of the hash. TUN module keeps returned hash type in
> (currently unused) field of the skb.
> skb->__unused renamed to 'hash_report_type'.
>
> When TUN module is called later to allocate and fill the virtio-net
> header and push it to destination virtqueue it populates the hash
> and the hash type into virtio-net header.
>
> VHOST driver is made aware of respective virtio-net feature that
> extends the virtio-net header to report the hash value and hash report
> type.
>
> Yuri Benditovich (7):
>   skbuff: define field for hash report type
>   vhost: support for hash report virtio-net feature
>   tun: allow use of BPF_PROG_TYPE_SCHED_CLS program type
>   tun: free bpf_program by bpf_prog_put instead of bpf_prog_destroy
>   tun: add ioctl code TUNSETHASHPOPULATION
>   tun: populate hash in virtio-net header when needed
>   tun: report new tun feature IFF_HASH

Patch 1/7 is missing.

Skbuff fields are in short supply. I don't think we need to add one
just for this narrow path entirely internal to the tun device.

Instead, you could just run the flow_dissector in tun_put_user if the
feature is negotiated. Indeed, the flow dissector seems more apt to me
than BPF here. Note that the flow dissector internally can be
overridden by a BPF program if the admin so chooses.

This also hits on a deeper point with the choice of hash values, that
I also noticed in my RFC patchset to implement the inverse [1][2]. It
is much more detailed than skb->hash + skb->l4_hash currently offers,
and that can be gotten for free from most hardware. In most practical
cases, that information suffices. I added less specific fields
VIRTIO_NET_HASH_REPORT_L4, VIRTIO_NET_HASH_REPORT_OTHER that work
without explicit flow dissection. I understand that the existing
fields are part of the standard. Just curious, what is their purpose
beyond 4-tuple based flow hashing?

[1] https://patchwork.kernel.org/project/netdevbpf/list/?series=406859&state=*
[2] https://github.com/wdebruij/linux/commit/0f77febf22cd6ffc242a575807fa8382a26e511e