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 07:37:06 -0500

On Wed, Feb 25, 2026 at 12:34:28PM +0000, Srujana Challa wrote:
> > > > On Tue, Feb 24, 2026 at 12:28:49PM +0530, Srujana Challa wrote:
> > > > > Replace hardcoded RSS max key size limit with NETDEV_RSS_KEY_LEN
> > > > > to align with kernel's standard RSS key length. Add validation for
> > > > > RSS key size against spec minimum (40 bytes) and driver maximum.
> > > > > When validation fails, gracefully disable RSS features and
> > > > > continue initialization rather than failing completely.
> > > > >
> > > > > Cc: [email-masked]
> > > > > Fixes: 3f7d9c1964fc ("virtio_net: Add hash_key_length check")
> > > > > Signed-off-by: Srujana Challa <[email-masked]>
> > > >
> > > > --- should come here before changelog.
> > > >
> > > > > v3:
> > > > > - Moved RSS key validation checks to virtnet_validate.
> > > > > - Add fixes: tag and CC -stable
> > > > > v4:
> > > > > - Use NETDEV_RSS_KEY_LEN instead of type_max for the maximum rss
> > > > > key
> > > > size.
> > > > > ---
> > > > >  drivers/net/virtio_net.c | 34 ++++++++++++++++++++++++----------
> > > > >  1 file changed, 24 insertions(+), 10 deletions(-)
> > > > >
> > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > > index
> > > > > db88dcaefb20..eeefe8abc122 100644
> > > > > --- a/drivers/net/virtio_net.c
> > > > > +++ b/drivers/net/virtio_net.c
> > > > > @@ -381,8 +381,6 @@ struct receive_queue {
> > > > >  	struct xdp_buff **xsk_buffs;
> > > > >  };
> > > > >
> > > > > -#define VIRTIO_NET_RSS_MAX_KEY_SIZE     40
> > > > > -
> > > > >  /* Control VQ buffers: protected by the rtnl lock */  struct
> > > > > control_buf {
> > > > >  	struct virtio_net_ctrl_hdr hdr;
> > > > > @@ -486,7 +484,7 @@ struct virtnet_info {
> > > > >
> > > > >  	/* Must be last as it ends in a flexible-array member. */
> > > > >  	TRAILING_OVERLAP(struct virtio_net_rss_config_trailer,
> > > > > rss_trailer,
> > > > hash_key_data,
> > > > > -		u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];
> > > > > +		u8 rss_hash_key_data[NETDEV_RSS_KEY_LEN];
> > > > >  	);
> > > > >  };
> > > > >  static_assert(offsetof(struct virtnet_info,
> > > > > rss_trailer.hash_key_data) == @@ -6627,6 +6625,29 @@ static int
> > > > virtnet_validate(struct virtio_device *vdev)
> > > > >  		__virtio_clear_bit(vdev, VIRTIO_NET_F_STANDBY);
> > > > >  	}
> > > > >
> > > > > +	if (virtio_has_feature(vdev, VIRTIO_NET_F_RSS) ||
> > > > > +	    virtio_has_feature(vdev, VIRTIO_NET_F_HASH_REPORT)) {
> > > > > +		u8 key_sz = virtio_cread8(vdev,
> > > > > +					  offsetof(struct virtio_net_config,
> > > > > +						   rss_max_key_size));
> > > > > +		/* Spec requires at least 40 bytes */ #define
> > > > > +VIRTIO_NET_RSS_MIN_KEY_SIZE 40
> > > > > +		if (key_sz < VIRTIO_NET_RSS_MIN_KEY_SIZE) {
> > > > > +			dev_warn(&vdev->dev,
> > > > > +				 "rss_max_key_size=%u is less than spec
> > > > minimum %u, disabling RSS\n",
> > > > > +				 key_sz, VIRTIO_NET_RSS_MIN_KEY_SIZE);
> > > > > +			__virtio_clear_bit(vdev, VIRTIO_NET_F_RSS);
> > > > > +			__virtio_clear_bit(vdev,
> > > > VIRTIO_NET_F_HASH_REPORT);
> > > > > +		}
> > > > > +		if (key_sz > NETDEV_RSS_KEY_LEN) {
> > > > > +			dev_warn(&vdev->dev,
> > > > > +				 "rss_max_key_size=%u exceeds driver limit
> > > > %u, disabling RSS\n",
> > > > > +				 key_sz, NETDEV_RSS_KEY_LEN);
> > > > > +			__virtio_clear_bit(vdev, VIRTIO_NET_F_RSS);
> > > > > +			__virtio_clear_bit(vdev,
> > > > VIRTIO_NET_F_HASH_REPORT);
> > > >
> > > > you flipped the logic here and it makes no sense now.
> > > >
> > > > Did you test this path?
> > > Yes, tested with Marvell's Octeon device.
> > > >
> > > >
> > > > 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.

> > 
> > > >
> > > >
> > > > > +		}
> > > > > +	}
> > > > > +
> > > > >  	return 0;
> > > > >  }
> > > > >
> > > > > @@ -6839,13 +6860,6 @@ static int virtnet_probe(struct
> > > > > virtio_device
> > > > *vdev)
> > > > >  	if (vi->has_rss || vi->has_rss_hash_report) {
> > > > >  		vi->rss_key_size =
> > > > >  			virtio_cread8(vdev, offsetof(struct virtio_net_config,
> > > > rss_max_key_size));
> > > > > -		if (vi->rss_key_size > VIRTIO_NET_RSS_MAX_KEY_SIZE) {
> > > > > -			dev_err(&vdev->dev, "rss_max_key_size=%u exceeds
> > > > the limit %u.\n",
> > > > > -				vi->rss_key_size,
> > > > VIRTIO_NET_RSS_MAX_KEY_SIZE);
> > > > > -			err = -EINVAL;
> > > > > -			goto free;
> > > > > -		}
> > > > > -
> > > > >  		vi->rss_hash_types_supported =
> > > > >  		    virtio_cread32(vdev, offsetof(struct virtio_net_config,
> > > > supported_hash_types));
> > > > >  		vi->rss_hash_types_supported &=
> > > > > --
> > > > > 2.25.1
> > >
>