netdev - [RFC PATCH net] virtio_net: Prevent napi_weight changes with VIRTIO_NET_F_NOTF

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Date: Mon, 5 Jun 2023 14:02:36 -0700
From: Brett Creeley <brett.creeley@....com>
To: <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
	<virtualization@...ts.linux-foundation.org>, <alvaro.karsz@...id-run.com>,
	<pabeni@...hat.com>, <kuba@...nel.org>, <edumazet@...gle.com>,
	<davem@...emloft.net>, <xuanzhuo@...ux.alibaba.com>, <jasowang@...hat.com>,
	<mst@...hat.com>
CC: <brett.creeley@....com>, <shannon.nelson@....com>
Subject: [RFC PATCH net] virtio_net: Prevent napi_weight changes with VIRTIO_NET_F_NOTF_COAL support

Commit 699b045a8e43 ("net: virtio_net: notifications coalescing
support") added support for VIRTIO_NET_F_NOTF_COAL. The get_coalesce
call made changes to report "1" in tx_max_coalesced_frames if
VIRTIO_NET_F_NOTF_COAL is not supported and napi.weight is non-zero.
However, the napi_weight value could still be changed by the
set_coalesce call regardless of whether or not the device supports
VIRTIO_NET_F_NOTF_COAL.

It seems like the tx_max_coalesced_frames value should not control more
than 1 thing (i.e. napi_weight and the device's tx_max_packets). So, fix
this by only allowing the napi_weight change if VIRTIO_NET_F_NOTF_COAL
is not supported by the virtio device.

It wasn't clear to me if this was the intended behavior, so that's why
I'm sending this as an RFC patch initially. Based on the feedback, I
will resubmit as an official patch.

Fixes: 699b045a8e43 ("net: virtio_net: notifications coalescing support")
Signed-off-by: Brett Creeley <brett.creeley@....com>
---
 drivers/net/virtio_net.c | 24 +++++++++++++-----------
 1 file changed, 13 insertions(+), 11 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 486b5849033d..e28387866909 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2990,19 +2990,21 @@ static int virtnet_set_coalesce(struct net_device *dev,
 	int ret, i, napi_weight;
 	bool update_napi = false;

-	/* Can't change NAPI weight if the link is up */
-	napi_weight = ec->tx_max_coalesced_frames ? NAPI_POLL_WEIGHT : 0;
-	if (napi_weight ^ vi->sq[0].napi.weight) {
-		if (dev->flags & IFF_UP)
-			return -EBUSY;
-		else
-			update_napi = true;
-	}
-
-	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_NOTF_COAL))
+	if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_NOTF_COAL)) {
 		ret = virtnet_send_notf_coal_cmds(vi, ec);
-	else
+	} else {
+		/* Can't change NAPI weight if the link is up */
+		napi_weight = ec->tx_max_coalesced_frames ?
+			NAPI_POLL_WEIGHT : 0;
+		if (napi_weight ^ vi->sq[0].napi.weight) {
+			if (dev->flags & IFF_UP)
+				return -EBUSY;
+			else
+				update_napi = true;
+		}
+
 		ret = virtnet_coal_params_supported(ec);
+	}

 	if (ret)
 		return ret;
-- 
2.17.1