Tested this patch with virtio-net regression tests, everything works fine.

Tested-by: Lei Yang leiyang@redhat.com

On Fri, Oct 31, 2025 at 2:06 PM Jason Wang jasowang@redhat.com wrote:

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

---

Changes since V1:

• Fix build issues of virtio_net_hdr_tnl_from_skb()

---

drivers/net/virtio_net.c | 15 +++++++++++++-- include/linux/virtio_net.h | 3 ++- include/uapi/linux/virtio_net.h | 3 ++- 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index 8e8a179aaa49..e6e650bc3bc3 100644 --- a/drivers/net/virtio_net.c +++ b/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;
  

}

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h index 4d1780848d0e..b673c31569f3 100644 --- a/include/linux/virtio_net.h +++ b/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;
  
  

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

2.31.1

On Fri, Oct 31, 2025 at 02:05:51PM +0800, Jason Wang wrote:

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h index 4d1780848d0e..b673c31569f3 100644 --- a/include/linux/virtio_net.h +++ b/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;

BTW is it just me or is old code space-indented here? We should probably switch it to tabs.