On Fri, Sep 27, 2024 at 3:51 PM Akihiko Odaki akihiko.odaki@daynix.com wrote:
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.
Fair enough.
Btw, I remember DPDK implements tuntap RSS via eBPF as well (probably via cls or other). It might worth to see if anything we miss here.
Thanks
Regards, Akihiko Odaki