netdev - Re: [PATCH RFC v3 0/9] tun: Introduce virtio-net hashing feature

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Message-ID: <e59f0525-4f3b-4f9c-a359-659fd47dc385@daynix.com>
Date: Mon, 23 Sep 2024 19:57:56 +0200
From: Akihiko Odaki <akihiko.odaki@...nix.com>
To: Stephen Hemminger <stephen@...workplumber.org>
Cc: Jonathan Corbet <corbet@....net>,
 Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Jason Wang <jasowang@...hat.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>
Subject: Re: [PATCH RFC v3 0/9] tun: Introduce virtio-net hashing feature

On 2024/09/15 21:48, Stephen Hemminger wrote:
> On Sun, 15 Sep 2024 10:17:39 +0900
> 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).
> 
> This will be useful for DPDK. But there still are cases where custom
> flow rules are needed. I.e the RSS happens after other TC rules.
> It would be a good if skbedit supported RSS as an option.

Hi,

It is nice to hear about a use case other than QEMU or virtualization. I 
implemented RSS as tuntap ioctl because:
- It is easier to configure for the user of tuntap (e.g., QEMU)
- It implements hash reporting, which is specific to tuntap.

You can still add skbedit if you want to override RSS for some packets 
with filter. Please tell me if it is not sufficient for your use case.

Regards,
Akihiko Odaki