netdev - Re: [PATCH net v2 3/5] virtio-net: execute xdp_do_flush() before napi_complete

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Date:   Fri, 27 Jan 2023 05:49:47 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     magnus.karlsson@...el.com, bjorn@...nel.org, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org,
        jonathan.lemon@...il.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, toke@...hat.com, pabeni@...hat.com,
        davem@...emloft.net, aelior@...vell.com, manishc@...vell.com,
        horatiu.vultur@...rochip.com, UNGLinuxDriver@...rochip.com,
        jasowang@...hat.com, ioana.ciornei@....com, madalin.bucur@....com,
        bpf@...r.kernel.org
Subject: Re: [PATCH net v2 3/5] virtio-net: execute xdp_do_flush() before
 napi_complete_done()

On Wed, Jan 25, 2023 at 08:48:59AM +0100, Magnus Karlsson wrote:
> From: Magnus Karlsson <magnus.karlsson@...el.com>
> 
> Make sure that xdp_do_flush() is always executed before
> napi_complete_done(). This is important for two reasons. First, a
> redirect to an XSKMAP assumes that a call to xdp_do_redirect() from
> napi context X on CPU Y will be followed by a xdp_do_flush() from the
> same napi context and CPU. This is not guaranteed if the
> napi_complete_done() is executed before xdp_do_flush(), as it tells
> the napi logic that it is fine to schedule napi context X on another
> CPU. Details from a production system triggering this bug using the
> veth driver can be found following the first link below.
> 
> The second reason is that the XDP_REDIRECT logic in itself relies on
> being inside a single NAPI instance through to the xdp_do_flush() call
> for RCU protection of all in-kernel data structures. Details can be
> found in the second link below.
> 
> Fixes: 186b3c998c50 ("virtio-net: support XDP_REDIRECT")
> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
> Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>
> Link: https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@cloudflare.com
> Link: https://lore.kernel.org/all/20210624160609.292325-1-toke@redhat.com/

Acked-by: Michael S. Tsirkin <mst@...hat.com>

> ---
>  drivers/net/virtio_net.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 18b3de854aeb..6df14dd5bf46 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1677,13 +1677,13 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
>  
>  	received = virtnet_receive(rq, budget, &xdp_xmit);
>  
> +	if (xdp_xmit & VIRTIO_XDP_REDIR)
> +		xdp_do_flush();
> +
>  	/* Out of packets? */
>  	if (received < budget)
>  		virtqueue_napi_complete(napi, rq->vq, received);
>  
> -	if (xdp_xmit & VIRTIO_XDP_REDIR)
> -		xdp_do_flush();
> -
>  	if (xdp_xmit & VIRTIO_XDP_TX) {
>  		sq = virtnet_xdp_get_sq(vi);
>  		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
> -- 
> 2.34.1