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

Akihiko Odaki
2024-09-27T07:50:59Z

On 2024/09/27 13:31, Jason Wang wrote:
> 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]?

I didn't opt to using cb. Below is the rationale:

cb is for tail call so it means we reuse the field for a different
purpose. The context rewrite allows adding a field without increasing
the size of the underlying storage (the real sk_buff) so we should add a
new field instead of reusing an existing field to avoid confusion.

We are however no longer allowed to add a new field. In my
understanding, this is because it is an UAPI, and eBPF maintainers found
it is difficult to maintain its stability.

Reusing cb for hash reporting is a workaround to avoid having a new
field, but it does not solve the underlying problem (i.e., keeping eBPF
as stable as UAPI is unreasonably hard). In my opinion, adding an ioctl
is a reasonable option to keep the API as stable as other virtualization
UAPIs while respecting the underlying intention of the context rewrite
feature freeze.

Regards,
Akihiko Odaki