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From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date:   Sun, 8 Oct 2023 20:36:56 +0200
Message-ID: <CAF=yD-LnqYQ2qdiV+oygfHq5ZQb7QaSZ81XikzKjY8unfCTGRQ@mail.gmail.com>
Subject: Re: [RFC PATCH 0/7] tun: Introduce virtio-net hashing feature
To:     Akihiko Odaki <akihiko.odaki@daynix.com>
Cc:     Jason Wang <jasowang@redhat.com>,
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On Sun, Oct 8, 2023 at 7:21=E2=80=AFAM Akihiko Odaki <akihiko.odaki@daynix.=
com> wrote:
>
> virtio-net have two usage of hashes: one is RSS and another is hash
> reporting. Conventionally the hash calculation was done by the VMM.
> However, computing the hash after the queue was chosen defeats the
> purpose of RSS.
>
> Another approach is to use eBPF steering program. This approach has
> another downside: it cannot report the calculated hash due to the
> restrictive nature of eBPF.
>
> Introduce the code to compute hashes to the kernel in order to overcome
> thse challenges.
>
> An alternative solution is to extend the eBPF steering program so that it
> will be able to report to the userspace, but it makes little sense to
> allow to implement different hashing algorithms with eBPF since the hash
> value reported by virtio-net is strictly defined by the specification.

But using the existing BPF steering may have the benefit of requiring
a lot less new code.

There is ample precedence for BPF programs that work this way. The
flow dissector comes to mind.