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Message-ID: <20251029012434.75576-1-jasowang@redhat.com>
Date: Wed, 29 Oct 2025 09:24:34 +0800
From: Jason Wang <jasowang@...hat.com>
To: mst@...hat.com,
jasowang@...hat.com,
xuanzhuo@...ux.alibaba.com,
eperezma@...hat.com,
andrew+netdev@...n.ch,
davem@...emloft.net,
edumazet@...gle.com,
kuba@...nel.org,
pabeni@...hat.com
Cc: virtualization@...ts.linux.dev,
netdev@...r.kernel.org,
linux-kernel@...r.kernel.org,
stable@...r.kernel.org
Subject: [PATCH net] virtio_net: fix alignment for virtio_net_hdr_v1_hash
From: "Michael S. Tsirkin" <mst@...hat.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@...r.kernel.org
Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
Signed-off-by: Jason Wang <jasowang@...hat.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
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