virtio_net: fix alignment for virtio_net_hdr_v1_hash

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: 56a06bd ("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>
Tested-by: Lei Yang <leiyang@redhat.com>
Link: https://patch.msgid.link/20251031060551.126-1-jasowang@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: mstsirkin

drivers/net/virtio_net.c

@@ -2539,6 +2539,13 @@ 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)
 {
@@ -2565,7 +2572,7 @@ 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,
@@ -3311,6 +3318,10 @@ 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;
@@ -6750,7 +6761,7 @@ 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;
 }
 

include/linux/virtio_net.h

@@ -401,7 +401,8 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
 	if (!tnl_hdr_negotiated)
 		return -EINVAL;
 
-        vhdr->hash_hdr.hash_value = 0;
+	vhdr->hash_hdr.hash_value_lo = 0;
+	vhdr->hash_hdr.hash_value_hi = 0;
         vhdr->hash_hdr.hash_report = 0;
         vhdr->hash_hdr.padding = 0;
 

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