Re: [PATCH net] virtio_net: fix alignment for virtio_net_hdr_v1_hash

On Wed, Oct 29, 2025 at 4:20 PM Paolo Abeni [off-list ref] wrote:

On 10/29/25 2:24 AM, Jason Wang wrote:

quoted

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 <redacted>

Whoops, I replied to the older thread before reading this one.

Acked-by: Paolo Abeni <pabeni@redhat.com>

I apologize, build will be broken since

commit b2284768c6b32aa224ca7d0ef0741beb434f03aa
Author: Jason Wang [off-list ref]
Date:   Wed Oct 22 11:44:21 2025 +0800

    virtio-net: zero unused hash fields

I will prepare a new version.

Btw, it looks like there's an uAPI change that may break builds of the
userspace:

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;

We can have a kernel only version for this but it probably means we
need a kernel only version for all the future extension of vnet
header?

Thanks