netdev - Re: [PATCH net-next v2 2/3] vhost_net: Call peek

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Date: Thu, 29 Feb 2024 11:48:43 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Yunjian Wang <wangyunjian@...wei.com>, mst@...hat.com, 
	willemdebruijn.kernel@...il.com, jasowang@...hat.com, kuba@...nel.org, 
	bjorn@...nel.org, magnus.karlsson@...el.com, maciej.fijalkowski@...el.com, 
	jonathan.lemon@...il.com, davem@...emloft.net
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org,  kvm@...r.kernel.org,
 virtualization@...ts.linux.dev, xudingke@...wei.com,  liwei395@...wei.com
Subject: Re: [PATCH net-next v2 2/3] vhost_net: Call peek_len when using xdp

On Wed, 2024-02-28 at 19:05 +0800, Yunjian Wang wrote:
> If TUN supports AF_XDP TX zero-copy, the XDP program will enqueue
> packets to the XDP ring and wake up the vhost worker. This requires
> the vhost worker to call peek_len(), which can be used to consume
> XDP descriptors.
> 
> Signed-off-by: Yunjian Wang <wangyunjian@...wei.com>
> ---
>  drivers/vhost/net.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index f2ed7167c848..077e74421558 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -207,6 +207,11 @@ static int vhost_net_buf_peek_len(void *ptr)
>  	return __skb_array_len_with_tag(ptr);
>  }
>  
> +static bool vhost_sock_xdp(struct socket *sock)
> +{
> +	return sock_flag(sock->sk, SOCK_XDP);
> +}
> +
>  static int vhost_net_buf_peek(struct vhost_net_virtqueue *nvq)
>  {
>  	struct vhost_net_buf *rxq = &nvq->rxq;
> @@ -214,6 +219,13 @@ static int vhost_net_buf_peek(struct vhost_net_virtqueue *nvq)
>  	if (!vhost_net_buf_is_empty(rxq))
>  		goto out;
>  
> +	if (ptr_ring_empty(nvq->rx_ring)) {
> +		struct socket *sock = vhost_vq_get_backend(&nvq->vq);
> +		/* Call peek_len to consume XSK descriptors, when using xdp */
> +		if (vhost_sock_xdp(sock) && sock->ops->peek_len)
> +			sock->ops->peek_len(sock);

This really looks like a socket API misuse. Why can't you use ptr-ring
primitives to consume XSK descriptors? peek_len could be constified
some day, this code will prevent such (good) thing.

Cheers,

Paolo