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

[Akihiko Odaki <akihiko.odaki@daynix.com>]

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.

Regards,
Akihiko Odaki