[PATCH net] virtio_net: fix alignment for virtio_net_hdr_v1_hash - Linux-stable-mirror

29 Oct 2025

From: "Michael S. Tsirkin" mst@redhat.com
Changing alignment of header would mean it's no longer safe to cast a
2 byte aligned pointer between formats. Use two 16 bit fields to make
it 2 byte aligned as previously.
This fixes the performance regression since
commit ("virtio_net: enable gso over UDP tunnel support.") as it uses
virtio_net_hdr_v1_hash_tunnel which embeds
virtio_net_hdr_v1_hash. Pktgen in guest + XDP_DROP on TAP + vhost_net
shows the TX PPS is recovered from 2.4Mpps to 4.45Mpps.
Fixes: 56a06bd40fab ("virtio_net: enable gso over UDP tunnel support.")
Cc: stable@vger.kernel.org
Signed-off-by: Michael S. Tsirkin mst@redhat.com
Signed-off-by: Jason Wang jasowang@redhat.com
---
 drivers/net/virtio_net.c        | 15 +++++++++++++--
 include/uapi/linux/virtio_net.h |  3 ++-
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index a757cbcab87f..5e998d88db44 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2534,6 +2534,13 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
    return NULL;
 }
+static inline u32
+virtio_net_hash_value(const struct virtio_net_hdr_v1_hash *hdr_hash)
+{
+	return __le16_to_cpu(hdr_hash->hash_value_lo) |
+		(__le16_to_cpu(hdr_hash->hash_value_hi) << 16);
+}
+
 static void virtio_skb_set_hash(const struct virtio_net_hdr_v1_hash *hdr_hash,
    			struct sk_buff *skb)
 {
@@ -2560,7 +2567,7 @@ static void virtio_skb_set_hash(const struct virtio_net_hdr_v1_hash *hdr_hash,
    default:
    	rss_hash_type = PKT_HASH_TYPE_NONE;
    }
-	skb_set_hash(skb, __le32_to_cpu(hdr_hash->hash_value), rss_hash_type);
+	skb_set_hash(skb, virtio_net_hash_value(hdr_hash), rss_hash_type);
 }
static void virtnet_receive_done(struct virtnet_info *vi, struct receive_queue *rq,
@@ -3306,6 +3313,10 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb, bool orphan)
pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest);
+	/* Make sure it's safe to cast between formats */
+	BUILD_BUG_ON(__alignof__(*hdr) != __alignof__(hdr->hash_hdr));
+	BUILD_BUG_ON(__alignof__(*hdr) != __alignof__(hdr->hash_hdr.hdr));
+
    can_push = vi->any_header_sg &&
    	!((unsigned long)skb->data & (__alignof__(*hdr) - 1)) &&
    	!skb_header_cloned(skb) && skb_headroom(skb) >= hdr_len;
@@ -6745,7 +6756,7 @@ static int virtnet_xdp_rx_hash(const struct xdp_md *_ctx, u32 *hash,
    	hash_report = VIRTIO_NET_HASH_REPORT_NONE;
*rss_type = virtnet_xdp_rss_type[hash_report];
-	*hash = __le32_to_cpu(hdr_hash->hash_value);
+	*hash = virtio_net_hash_value(hdr_hash);
    return 0;
 }
diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
index 8bf27ab8bcb4..1db45b01532b 100644
--- a/include/uapi/linux/virtio_net.h
+++ b/include/uapi/linux/virtio_net.h
@@ -193,7 +193,8 @@ struct virtio_net_hdr_v1 {
struct virtio_net_hdr_v1_hash {
    struct virtio_net_hdr_v1 hdr;
-	__le32 hash_value;
+	__le16 hash_value_lo;
+	__le16 hash_value_hi;
 #define VIRTIO_NET_HASH_REPORT_NONE            0
 #define VIRTIO_NET_HASH_REPORT_IPv4            1
 #define VIRTIO_NET_HASH_REPORT_TCPv4           2
-- 
2.42.0



    