Re: [PATCH] Support for segment offloading on software interfaces for packets from virtual machine guests without the SKB_GSO_UDP_L4 flag.

[Willem de Bruijn]

echken wrote:
> When running virtual machines on a host, and the guest uses a kernel
> version below v6.2 (without commit https://
> github.com/torvalds/linux/commit/860b7f27b8f78564ca5a2f607e0820b2d352a562),

Prefer format commit 860b7f27b8f7 ("linux/virtio_net.h: Support USO
offload in vnet header.")

>  the UDP packets emitted from the guest do not include the SKB_GSO_UDP_L4
> flag in their skb gso_type. Therefore, UDP packets from such guests always
> bypass the __udp_gso_segment during the udp4_ufo_fragment process and go
> directly to software segmentation prematurely.

GSO packets should have either SKB_GSO_UDP_L4 (UDP segmentation) or
SKB_GSO_UDP (UDP fragmentation). Not both. Note that UFO is also long
deprecated and discouraged.

> When the guest sends UDP
> packets significantly larger than the MSS, and there are software
> interfaces in the data path, such as Geneve, this can lead to substantial
> additional performance overhead.
> 
> Signed-off-by: echken <chengcheng.luo@xxxxxxxxxx>
> ---
>  net/ipv4/udp_offload.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> index 59448a2dbf2c..6aa5a97d8bde 100644
> --- a/net/ipv4/udp_offload.c
> +++ b/net/ipv4/udp_offload.c
> @@ -402,6 +402,13 @@ static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb,
>  	if (unlikely(skb->len <= mss))
>  		goto out;
>  
> +	if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
> +		/* Packet is from an untrusted source, reset gso_segs. */
> +		skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len - sizeof(*uh),
> +							 mss);
> +		return NULL;
> +	}
> +

So what this really does is bypass software fragmentation in virtual
devices that advertise SKB_GSO_UDP.

That's fine, I suppose. But is not what the commit message currently
says.

>  	/* Do software UFO. Complete and fill in the UDP checksum as
>  	 * HW cannot do checksum of UDP packets sent as multiple
>  	 * IP fragments.
> -- 
> 2.34.1
>