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

On Tue, Jan 12, 2021 at 9:41 PM Yuri Benditovich
[off-list ref] 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.

Comment from Willem de Bruijn:

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

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

 drivers/net/tun.c           | 43 +++++++++++++++++++++++++++++++------
 drivers/vhost/net.c         | 37 ++++++++++++++++++++++++-------
 include/linux/skbuff.h      |  7 +++++-
 include/uapi/linux/if_tun.h |  2 ++
 4 files changed, 74 insertions(+), 15 deletions(-)

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2.17.1