Subject: Re: [EXTERNAL] Re: [PATCH net,v4,1/2] virtio_net: Improve RSS key size validation and use NETDEV_RSS_KEY_LEN

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 25 Feb 2026 08:57:43 -0500

On Wed, Feb 25, 2026 at 01:31:50PM +0000, Srujana Challa wrote:
> > On Wed, Feb 25, 2026 at 12:56:19PM +0000, Srujana Challa wrote:
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > So if device is powerful and supports a very big key size then...
> > > > > > > > > we disable the feature? how does this make sense?
> > > > > > > > The intent isn’t to disable the feature on capable devices,
> > > > > > > > but to ensure the driver never advertises support for RSS
> > > > > > > > key sizes larger than what the net device can actually
> > > > > > > > handle. Even if a device reports a very
> > > > > > > large key size, the driver is constrained by
> > > > > > > NETDEV_RSS_KEY_LEN, since
> > > > > > > netdev_rss_key_fill() enforces:
> > > > > > > > BUG_ON(len > sizeof(netdev_rss_key));
> > > > > > >
> > > > > > > so cap it to NETDEV_RSS_KEY_LEN. Why is that a reason to clear
> > > > > > > the
> > > > > feature?
> > > > > > Our device mandates that hash_key_length must be identical to
> > > > > > rss_max_key_size to guarantee symmetric bidirectional flow hashing.
> > > > > > If rss_max_key_size is larger than VIRTIO_NET_RSS_MAX_KEY_SIZE,
> > > > > > clamping
> > > > > the value is not feasible.
> > > > >
> > > > > I don't know what to tell you. rss_max_key_size is just the max
> > > > > device supports. driver should be free to use a smaller size.
> > > > My understanding is that this patch prevents the probe from failing
> > > > by disabling the feature instead.
> > > > Given the current implementation, the driver becomes unusable when
> > > > this condition is hit.
> > >
> > > I understand that the driver is allowed to use a smaller RSS key than the
> > device’s advertised rss_max_key_size.
> > > But, our hardware does not behave correctly in that configuration. For
> > > symmetric bidirectional hashing, the device requires that the
> > hash_key_length match rss_max_key_size exactly.
> > > If the driver uses a smaller key, the hardware produces inconsistent hash
> > values for forward vs reverse flows.
> > > Because of this device requirement, we cannot cap the key to
> > > NETDEV_RSS_KEY_LEN when the device advertises a larger
> > rss_max_key_size.
> > 
> > Would you not say it's a buggy device then?
> No. The device works correctly when a smaller key is used. The limitation only affects
> symmetric bidirectional hashing. For the other use cases capping the key size is fine.

yes the device seems buggy in that configuration, in that it has this
requirement which does not seem to be in the spec.

so tell me, what is the actual rss_max_key_size of that device?

> > --
> > MST