Subject: Re: [PATCH net] virtio-net: calculate header alignment mask based on features

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 28 Oct 2025 10:38:24 -0400

On Tue, Oct 28, 2025 at 11:03:41AM +0800, Jason Wang wrote:
> Commit 56a06bd40fab ("virtio_net: enable gso over UDP tunnel support.")
> switches to check the alignment of the virtio_net_hdr_v1_hash_tunnel
> even when doing the transmission even if the feature is not
> negotiated. This will cause 

you mean this causes

>a series performance degradation of pktgen
> as the skb->data can't satisfy the alignment requirement due to the
> increase of the header size then virtio-net must prepare at least 2
> sgs with indirect descriptors which will introduce overheads in the

introduces, accordinglt

> device.
> 
> Fixing this by calculate the header alignment during probe so when
> tunnel gso is not negotiated, we can less strict.
> 
> Pktgen in guest + XDP_DROP on TAP + vhost_net shows the TX PPS is
> recovered from 2.4Mpps to 4.45Mpps.
> 
> Note that we still need a way to recover the performance when tunnel
> gso is enabled, probably a new vnet header format.

you mean improve, not recover as such

> 
> Fixes: 56a06bd40fab ("virtio_net: enable gso over UDP tunnel support.")
> Cc: [email-masked]
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/net/virtio_net.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 31bd32bdecaf..5b851df749c0 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -441,6 +441,9 @@ struct virtnet_info {
>  	/* Packet virtio header size */
>  	u8 hdr_len;
>  
> +	/* header alignment */
> +	size_t hdr_align;
> +

It makes no sense to have u8 for length but size_t for alignment,
and u8 would fit in a memory hole we have, anyway.

>  	/* Work struct for delayed refilling if we run low on memory. */
>  	struct delayed_work refill;
>  
> @@ -3308,8 +3311,9 @@ 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);
>  
>  	can_push = vi->any_header_sg &&
> -		!((unsigned long)skb->data & (__alignof__(*hdr) - 1)) &&
> +		!((unsigned long)skb->data & (vi->hdr_align - 1)) &&

So let me get it straight.
We use the alignment check to be able to cast to the correct type.
The issue is that alignment for the header changed. 

virtio_net_hdr_v1 has 2 byte alignment, but:

struct virtio_net_hdr_v1_hash {
        struct virtio_net_hdr_v1 hdr; 
        __le32 hash_value;
#define VIRTIO_NET_HASH_REPORT_NONE            0
#define VIRTIO_NET_HASH_REPORT_IPv4            1
#define VIRTIO_NET_HASH_REPORT_TCPv4           2
#define VIRTIO_NET_HASH_REPORT_UDPv4           3
#define VIRTIO_NET_HASH_REPORT_IPv6            4
#define VIRTIO_NET_HASH_REPORT_TCPv6           5
#define VIRTIO_NET_HASH_REPORT_UDPv6           6
#define VIRTIO_NET_HASH_REPORT_IPv6_EX         7
#define VIRTIO_NET_HASH_REPORT_TCPv6_EX        8
#define VIRTIO_NET_HASH_REPORT_UDPv6_EX        9
        __le16 hash_report; 
        __le16 padding;
};      

has 4 byte due to hash_value, and accordingly:

struct virtio_net_hdr_v1_hash_tunnel {
        struct virtio_net_hdr_v1_hash hash_hdr;
        __le16 outer_th_offset;                                
        __le16 inner_nh_offset;
};              

now is 4 byte aligned so everything is messed up:
net tends not to be 4 byte aligned.

>  		!skb_header_cloned(skb) && skb_headroom(skb) >= hdr_len;
> +
>  	/* Even if we can, don't push here yet as this would skew
>  	 * csum_start offset below. */
>  	if (can_push)
> @@ -6926,15 +6930,20 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	}
>  
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO) ||
> -	    virtio_has_feature(vdev, VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO))
> +	    virtio_has_feature(vdev, VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO)) {
>  		vi->hdr_len = sizeof(struct virtio_net_hdr_v1_hash_tunnel);
> -	else if (vi->has_rss_hash_report)
> +		vi->hdr_align = __alignof__(struct virtio_net_hdr_v1_hash_tunnel);
> +	} else if (vi->has_rss_hash_report) {
>  		vi->hdr_len = sizeof(struct virtio_net_hdr_v1_hash);
> -	else if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF) ||
> -		 virtio_has_feature(vdev, VIRTIO_F_VERSION_1))
> +		vi->hdr_align = __alignof__(struct virtio_net_hdr_v1_hash);
> +	} else if (virtio_has_feature(vdev, VIRTIO_NET_F_MRG_RXBUF) ||
> +		virtio_has_feature(vdev, VIRTIO_F_VERSION_1)) {
>  		vi->hdr_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
> -	else
> +		vi->hdr_align = __alignof__(struct virtio_net_hdr_mrg_rxbuf);
> +	} else {
>  		vi->hdr_len = sizeof(struct virtio_net_hdr);
> +		vi->hdr_align = __alignof__(struct virtio_net_hdr);
> +	}
>  
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_CSUM))
>  		vi->rx_tnl_csum = true;

So how about just fixing the root cause then?
Like this (untested, if you agree pls take over this):

---

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.

Signed-off-by: Michael S. Tsirkin <[email-masked]>

-- 

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 31bd32bdecaf..02ce5316f47d 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2535,6 +2535,13 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 	return NULL;
 }
 
+static inline u16
+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)
 {
@@ -2561,7 +2568,7 @@ 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,
@@ -3307,6 +3314,10 @@ 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;
@@ -6755,7 +6766,7 @@ 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