linux-kernel - Re: [PATCH net 1/2] vhost-net: unbreak busy polling

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Message-ID: <20250912045102-mutt-send-email-mst@kernel.org>
Date: Fri, 12 Sep 2025 04:51:08 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: eperezma@...hat.com, jonah.palmer@...cle.com, kuba@...nel.org,
	jon@...anix.com, kvm@...r.kernel.org,
	virtualization@...ts.linux.dev, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH net 1/2] vhost-net: unbreak busy polling

On Fri, Sep 12, 2025 at 04:26:57PM +0800, Jason Wang wrote:
> Commit 67a873df0c41 ("vhost: basic in order support") pass the number
> of used elem to vhost_net_rx_peek_head_len() to make sure it can
> signal the used correctly before trying to do busy polling. But it
> forgets to clear the count, this would cause the count run out of sync
> with handle_rx() and break the busy polling.
> 
> Fixing this by passing the pointer of the count and clearing it after
> the signaling the used.
> 
> Cc: stable@...r.kernel.org
> Fixes: 67a873df0c41 ("vhost: basic in order support")
> Signed-off-by: Jason Wang <jasowang@...hat.com>

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

> ---
>  drivers/vhost/net.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index c6508fe0d5c8..16e39f3ab956 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -1014,7 +1014,7 @@ static int peek_head_len(struct vhost_net_virtqueue *rvq, struct sock *sk)
>  }
>  
>  static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk,
> -				      bool *busyloop_intr, unsigned int count)
> +				      bool *busyloop_intr, unsigned int *count)
>  {
>  	struct vhost_net_virtqueue *rnvq = &net->vqs[VHOST_NET_VQ_RX];
>  	struct vhost_net_virtqueue *tnvq = &net->vqs[VHOST_NET_VQ_TX];
> @@ -1024,7 +1024,8 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk,
>  
>  	if (!len && rvq->busyloop_timeout) {
>  		/* Flush batched heads first */
> -		vhost_net_signal_used(rnvq, count);
> +		vhost_net_signal_used(rnvq, *count);
> +		*count = 0;
>  		/* Both tx vq and rx socket were polled here */
>  		vhost_net_busy_poll(net, rvq, tvq, busyloop_intr, true);
>  
> @@ -1180,7 +1181,7 @@ static void handle_rx(struct vhost_net *net)
>  
>  	do {
>  		sock_len = vhost_net_rx_peek_head_len(net, sock->sk,
> -						      &busyloop_intr, count);
> +						      &busyloop_intr, &count);
>  		if (!sock_len)
>  			break;
>  		sock_len += sock_hlen;
> -- 
> 2.34.1