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

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Message-ID: <CACGkMEsTd6uCOCre8HK=5G14zu+xVOPOORZ2xcV_n9Kg6w8F5Q@mail.gmail.com>
Date: Thu, 30 Oct 2025 10:40:13 +0800
From: Jason Wang <jasowang@...hat.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: mst@...hat.com, xuanzhuo@...ux.alibaba.com, eperezma@...hat.com, 
	andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com, 
	kuba@...nel.org, 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 Wed, Oct 29, 2025 at 4:20 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> 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>

I apologize, build will be broken since

commit b2284768c6b32aa224ca7d0ef0741beb434f03aa
Author: Jason Wang <jasowang@...hat.com>
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

>