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

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Message-ID: <CACGkMEu3u=_=PWW-=XavJRduiHJuZwv11OrMZbnBNVn1fptRUw@mail.gmail.com>
Date: Fri, 27 Sep 2024 12:31:23 +0800
From: Jason Wang <jasowang@...hat.com>
To: Akihiko Odaki <akihiko.odaki@...nix.com>
Cc: Jonathan Corbet <corbet@....net>, Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	"Michael S. Tsirkin" <mst@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, kvm@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org, linux-kselftest@...r.kernel.org, 
	Yuri Benditovich <yuri.benditovich@...nix.com>, Andrew Melnychenko <andrew@...nix.com>, 
	Stephen Hemminger <stephen@...workplumber.org>, gur.stavi@...wei.com
Subject: Re: [PATCH RFC v4 0/9] tun: Introduce virtio-net hashing feature

On Fri, Sep 27, 2024 at 10:11 AM Akihiko Odaki <akihiko.odaki@...nix.com> wrote:
>
> On 2024/09/25 12:30, Jason Wang wrote:
> > On Tue, Sep 24, 2024 at 5:01 PM Akihiko Odaki <akihiko.odaki@...nix.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
>