On Sun, Oct 8, 2023 at 7:21 AM 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.