[PATCH] virtio_net: normalize non-positive napi_weight values

[PATCH] virtio_net: normalize non-positive napi_weight values

[Samuel Moelius]

The virtio_net.napi_weight module parameter is signed and is copied
directly into the RX NAPI weight after netif_napi_add_config().

A value of -1 lets virtnet_poll() run with a negative budget.  The RX
loop processes no packets, the unsigned received-vs-budget comparison
completes NAPI, and the NAPI core reports:

NAPI poll function virtnet_poll returned 0, exceeding its budget of -1

The device then repeatedly drops receive progress under stock QEMU
virtio-net traffic.

Normalize non-positive values to the default NAPI weight before
assigning the RX and TX NAPI weights.  TX NAPI can still be disabled
through the separate napi_tx parameter, which intentionally sets the TX
weight to zero.

Assisted-by: Codex:gpt-5.5-cyber-preview
Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
---
 drivers/net/virtio_net.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index f4adcfee7a80..667026607cfa 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -6481,6 +6481,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
 static int virtnet_alloc_queues(struct virtnet_info *vi)
 {
 	int i;
+	int weight = napi_weight > 0 ? napi_weight : NAPI_POLL_WEIGHT;
 
 	if (vi->has_cvq) {
 		vi->ctrl = kzalloc_obj(*vi->ctrl);
@@ -6500,10 +6501,10 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
 		vi->rq[i].pages = NULL;
 		netif_napi_add_config(vi->dev, &vi->rq[i].napi, virtnet_poll,
 				      i);
-		vi->rq[i].napi.weight = napi_weight;
+		vi->rq[i].napi.weight = weight;
 		netif_napi_add_tx_weight(vi->dev, &vi->sq[i].napi,
 					 virtnet_poll_tx,
-					 napi_tx ? napi_weight : 0);
+					 napi_tx ? weight : 0);
 
 		sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
 		ewma_pkt_len_init(&vi->rq[i].mrg_avg_pkt_len);
-- 
2.43.0

Re: [PATCH] virtio_net: normalize non-positive napi_weight values

[Michael S. Tsirkin]

On Fri, Jun 05, 2026 at 07:40:44PM +0000, Samuel Moelius wrote:
> The virtio_net.napi_weight module parameter is signed and is copied
> directly into the RX NAPI weight after netif_napi_add_config().
> 
> A value of -1 lets virtnet_poll() run with a negative budget.  The RX
> loop processes no packets, the unsigned received-vs-budget comparison
> completes NAPI, and the NAPI core reports:
> 
> NAPI poll function virtnet_poll returned 0, exceeding its budget of -1
> 
> The device then repeatedly drops receive progress under stock QEMU
> virtio-net traffic.
> 
> Normalize non-positive values to the default NAPI weight before
> assigning the RX and TX NAPI weights.  TX NAPI can still be disabled
> through the separate napi_tx parameter, which intentionally sets the TX
> weight to zero.
> 
> Assisted-by: Codex:gpt-5.5-cyber-preview
> Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>


Why should we bother working around admin errors in the kernel?
Leave it for the userspace tooling which can at least provide
useful diagnostics.

> ---
>  drivers/net/virtio_net.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index f4adcfee7a80..667026607cfa 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -6481,6 +6481,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
>  static int virtnet_alloc_queues(struct virtnet_info *vi)
>  {
>  	int i;
> +	int weight = napi_weight > 0 ? napi_weight : NAPI_POLL_WEIGHT;
>  
>  	if (vi->has_cvq) {
>  		vi->ctrl = kzalloc_obj(*vi->ctrl);
> @@ -6500,10 +6501,10 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
>  		vi->rq[i].pages = NULL;
>  		netif_napi_add_config(vi->dev, &vi->rq[i].napi, virtnet_poll,
>  				      i);
> -		vi->rq[i].napi.weight = napi_weight;
> +		vi->rq[i].napi.weight = weight;
>  		netif_napi_add_tx_weight(vi->dev, &vi->sq[i].napi,
>  					 virtnet_poll_tx,
> -					 napi_tx ? napi_weight : 0);
> +					 napi_tx ? weight : 0);
>  
>  		sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
>  		ewma_pkt_len_init(&vi->rq[i].mrg_avg_pkt_len);
> -- 
> 2.43.0