netdev - Re: [PATCH RFC v4 7/9] tun: Introduce virtio-net RSS

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Message-ID: <20240924130550.GJ4029621@kernel.org>
Date: Tue, 24 Sep 2024 14:05:50 +0100
From: Simon Horman <horms@...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>,
	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 7/9] tun: Introduce virtio-net RSS

On Tue, Sep 24, 2024 at 11:01:12AM +0200, Akihiko Odaki wrote:
> 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.
> 
> Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>

...

> diff --git a/drivers/net/tun.c b/drivers/net/tun.c

...

> @@ -333,8 +336,10 @@ static long tun_set_vnet_be(struct tun_struct *tun, int __user *argp)
>  		return -EFAULT;
>  
>  	if (be) {
> +		struct tun_vnet_hash_container *vnet_hash = rtnl_dereference(tun->vnet_hash);
> +
>  		if (!(tun->flags & TUN_VNET_LE) &&
> -		    (tun->vnet_hash.flags & TUN_VNET_HASH_REPORT))
> +		    vnet_hash && (vnet_hash->flags & TUN_VNET_HASH_REPORT))

Hi Odaki-san,

I am wondering if the above should this be vnet_hash->common.flags?
I am seeing this:

../drivers/net/tun.c:342:44: error: ‘struct tun_vnet_hash_container’ has no member named ‘flags’
  342 |                     vnet_hash && (vnet_hash->flags & TUN_VNET_HASH_REPORT))

...