linux-kernel - Re: [RFC PATCH 0/7] tun: Introduce virtio-net hashing feature

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Message-ID: <CAF=yD-LnqYQ2qdiV+oygfHq5ZQb7QaSZ81XikzKjY8unfCTGRQ@mail.gmail.com>
Date:   Sun, 8 Oct 2023 20:36:56 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Akihiko Odaki <akihiko.odaki@...nix.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kselftest@...r.kernel.org, bpf@...r.kernel.org,
        davem@...emloft.net, kuba@...nel.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, rdunlap@...radead.org, willemb@...gle.com,
        gustavoars@...nel.org, herbert@...dor.apana.org.au,
        steffen.klassert@...unet.com, nogikh@...gle.com,
        pablo@...filter.org, decui@...rosoft.com, cai@....pw,
        jakub@...udflare.com, elver@...gle.com, pabeni@...hat.com,
        Yuri Benditovich <yuri.benditovich@...nix.com>
Subject: Re: [RFC PATCH 0/7] tun: Introduce virtio-net hashing feature

On Sun, Oct 8, 2023 at 7:21 AM Akihiko Odaki <akihiko.odaki@...nix.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.