RE: [EXTERNAL] [PATCH 6.12.y] virtio_net: clamp rss_max_key_size to NETDEV_RSS_KEY_LEN

Srujana Challa
2026-04-08T13:50:09Z

> From: Srujana Challa <schalla@marvell.com>
>
> [ Upstream commit b4e5f04c58a29c499faa85d12952ca9a4faf1cb9 ]
>
> rss_max_key_size in the virtio spec is the maximum key size supported by the
> device, not a mandatory size the driver must use. Also the value 40 is a spec
> minimum, not a spec maximum.
>
> The current code rejects RSS and can fail probe when the device reports a
> larger rss_max_key_size than the driver buffer limit. Instead, clamp the
> effective key length to min(device rss_max_key_size, NETDEV_RSS_KEY_LEN)
> and keep RSS enabled.
>
> This keeps probe working on devices that advertise larger maximum key sizes
> while respecting the netdev RSS key buffer size limit.
>
> Fixes: 3f7d9c1964fc ("virtio_net: Add hash_key_length check")
> Cc: stable@vger.kernel.org
> Signed-off-by: Srujana Challa <schalla@marvell.com>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> Link: https://urldefense.proofpoint.com/v2/url?u=https-
> 3A__patch.msgid.link_20260326142344.1171317-2D1-2Dschalla-
> 40marvell.com&d=DwIDAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=Fj4OoD5hcKFp
> ANhTWdwQzjT1Jpf7veC5263T47JVpnc&m=0XuKVXgk9_1LUIZHeqL0znGhAh
> x5KvAOLvrCl-orVeQSt__4o6Djr-79rwCl6KNp&s=cfQpAcZTTE7nTYku-
> MVkfip0xUJoBBw4ikqm9iEdgcc&e=
> Signed-off-by: Jakub Kicinski <kuba@kernel.org> [ changed clamp target from
> NETDEV_RSS_KEY_LEN to VIRTIO_NET_RSS_MAX_KEY_SIZE ]
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
> drivers/net/virtio_net.c | 16 ++++++++--------
> 1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index
> 5c83983f0eb3f..5a31ccdae2e22 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -6502,6 +6502,7 @@ static int virtnet_probe(struct virtio_device *vdev)
> struct virtnet_info *vi;
> u16 max_queue_pairs;
> int mtu = 0;
> + u16 key_sz;
>
> /* Find if host supports multiqueue/rss virtio_net device */
> max_queue_pairs = 1;
> @@ -6624,14 +6625,13 @@ static int virtnet_probe(struct virtio_device
> *vdev)
> goto free;
>
> 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;
> - }
> + key_sz = virtio_cread8(vdev, offsetof(struct virtio_net_config,
> +rss_max_key_size));
> +
> + vi->rss_key_size = min_t(u16, key_sz,
> VIRTIO_NET_RSS_MAX_KEY_SIZE);
> + if (key_sz > vi->rss_key_size)
> + dev_warn(&vdev->dev,
> + "rss_max_key_size=%u exceeds driver limit
> %u, clamping\n",
> + key_sz, vi->rss_key_size);
>
> vi->rss_hash_types_supported =
> virtio_cread32(vdev, offsetof(struct virtio_net_config,
> supported_hash_types));
> --
> 2.53.0

We used `NETDEV_RSS_KEY_LEN` intentionally for clamping.
`rss_max_key_size` is the maximum supported by the device,
while `40` is a spec minimum, not a maximum.
Clamping to `VIRTIO_NET_RSS_MAX_KEY_SIZE` would unnecessarily
limit valid devices(for example devices advertising 48/52 bytes) and
could reintroduce the original issue.

Could you please share the reason for changing the clamp target
from `NETDEV_RSS_KEY_LEN` to `VIRTIO_NET_RSS_MAX_KEY_SIZE`?

Thanks!