netdev - Re: [PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY

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Message-ID: <CANn89i+e+eawQVvmVuA7jyZ=DCU+wPq3z+q6_PoVVY7ZhX2UGQ@mail.gmail.com>
Date: Thu, 22 Jan 2026 17:17:52 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Srujana Challa <schalla@...vell.com>, Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, 
	virtualization@...ts.linux.dev, pabeni@...hat.com, mst@...hat.com, 
	jasowang@...hat.com, xuanzhuo@...ux.alibaba.com, eperezma@...hat.com, 
	andrew+netdev@...n.ch, davem@...emloft.net, ndabilpuram@...vell.com, 
	Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH net-next] virtio_net: Increase RSS max key size to match NETDEV_RSS_KEY_LEN

On Thu, Jan 22, 2026 at 5:13 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Thu, Jan 22, 2026 at 4:52 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Thu, 22 Jan 2026 14:45:27 +0530 Srujana Challa wrote:
> > > Increase VIRTIO_NET_RSS_MAX_KEY_SIZE from 40 to 52 bytes to align with
> > > the kernel's standard RSS key length defined by NETDEV_RSS_KEY_LEN.
> >
> > I've been meaning to bump NETDEV_RSS_KEY_LEN to 128 or some such.
> > Most modern drivers can't use netdev_rss_key_fill() either because
> > core generates a tiny key. And the devices support hashing over two
> > layers of IPv6 for tunnels.
>
> Seems fine to me. Back in 2014, RSS was limited to 52 bytes on all known NIC.
>

BTW make it 256 bytes. 128 is already too small.

> >
> > Willem, WDYT? Is there a reason to keep the core key small?
> > Or deprecate netdev_rss_key_fill()?
>
> What do you mean by deprecating it ?
>
> One of the ideas was to make sure multiple NIC would share the same key,
> for some bonding setups.
>
> commit 960fb622f85180f36d3aff82af53e2be3db2f888    net: provide a per
> host RSS key generic infrastructure
>
>
>
> >
> > > The virtio specification requires devices to support at least 40 bytes
> > > for the RSS key size. However, devices may support larger key sizes
> > > up to 52 bytes (as reported by the device's rss_max_key_size config
> > > field). This change allows such devices to work properly.
> > > Previously, devices reporting rss_max_key_size > 40 would fail with
> > > an error during initialization.
> > >
> > > The driver already handles variable key sizes dynamically through
> > > vi->rss_key_size, so increasing the maximum limit is safe and
> > > maintains backward compatibility with devices that support smaller
> > > key sizes.
> >
> > > -#define VIRTIO_NET_RSS_MAX_KEY_SIZE     40
> > > +#define VIRTIO_NET_RSS_MAX_KEY_SIZE     52