Re: [PATCH RFC v4 0/9] tun: Introduce virtio-net hashing feature

Jason Wang
2024-09-27T04:31:40Z

On Fri, Sep 27, 2024 at 10:11 AM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>
> On 2024/09/25 12:30, Jason Wang wrote:
> > On Tue, Sep 24, 2024 at 5:01 PM 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 is based on context
> >> rewrites, which is in feature freeze. We can adopt kfuncs, but they will
> >> not be UAPIs. We opt to ioctl to align with other relevant UAPIs (KVM
> >> and vhost_net).
> >>
> >
> > I wonder if we could clone the skb and reuse some to store the hash,
> > then the steering eBPF program can access these fields without
> > introducing full RSS in the kernel?
>
> I don't get how cloning the skb can solve the issue.
>
> We can certainly implement Toeplitz function in the kernel or even with
> tc-bpf to store a hash value that can be used for eBPF steering program
> and virtio hash reporting. However we don't have a means of storing a
> hash type, which is specific to virtio hash reporting and lacks a
> corresponding skb field.

I may miss something but looking at sk_filter_is_valid_access(). It
looks to me we can make use of skb->cb[0..4]?

Thanks

>
> Regards,
> Akihiko Odaki
>