netdev - Re: [PATCH net] virtio_net: fix alignment for virtio_net_hdr_v1

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Message-ID: <d0f1f8f5-8edf-4409-a3ee-376828f85618@redhat.com>
Date: Wed, 29 Oct 2025 09:19:57 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Jason Wang <jasowang@...hat.com>, mst@...hat.com,
 xuanzhuo@...ux.alibaba.com, eperezma@...hat.com, andrew+netdev@...n.ch,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org
Cc: virtualization@...ts.linux.dev, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH net] virtio_net: fix alignment for virtio_net_hdr_v1_hash

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

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

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