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

["Michael S. Tsirkin" <mst@redhat.com>]

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?

-- 
MST