linux-kernel - Re: [PATCH net-next v8 3/6] tun: Introduce virtio-net hash feature

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Message-ID: <20250306175550.03c90e21@kernel.org>
Date: Thu, 6 Mar 2025 17:55:50 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Akihiko Odaki <akihiko.odaki@...nix.com>
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>, 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, Lei Yang <leiyang@...hat.com>, Simon Horman
 <horms@...nel.org>
Subject: Re: [PATCH net-next v8 3/6] tun: Introduce virtio-net hash feature

On Thu, 06 Mar 2025 18:56:33 +0900 Akihiko Odaki wrote:
> Hash reporting
> ==============
> 
> Allow the guest to reuse the hash value to make receive steering
> consistent between the host and guest, and to save hash computation.
> 
> RSS
> ===
> 
> RSS is a receive steering algorithm that can be negotiated to use with
> virtio_net. 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 steering program.
> 
> Introduce the code to perform RSS 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 I didn't
> opt for it because extending the current mechanism of eBPF steering
> program as is because it relies on legacy context rewriting, and
> introducing kfunc-based eBPF will result in non-UAPI dependency while
> the other relevant virtualization APIs such as KVM and vhost_net are
> UAPIs.

drivers/net/tap.c:1056:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
 1056 |         case SIOCGIFHWADDR:
      |         ^
drivers/net/tap.c:1056:2: note: insert '__attribute__((fallthrough));' to silence this warning
 1056 |         case SIOCGIFHWADDR:
      |         ^
      |         __attribute__((fallthrough)); 
drivers/net/tap.c:1056:2: note: insert 'break;' to avoid fall-through
 1056 |         case SIOCGIFHWADDR:
      |         ^
      |         break; 
-- 
pw-bot: cr