Subject: Re: [PATCH] virtio_net: Do not send RSS key if it is not supported

From: Breno Leitao <[email-masked]>
Date: Mon, 25 Mar 2024 04:26:08 -0700

Hello Xuan,

On Mon, Mar 25, 2024 at 01:57:53PM +0800, Xuan Zhuo wrote:
> On Fri, 22 Mar 2024 03:21:21 -0700, Breno Leitao <[email-masked]> wrote:
> > Hello Xuan,
> >
> > On Fri, Mar 22, 2024 at 10:00:22AM +0800, Xuan Zhuo wrote:
> > > On Thu, 21 Mar 2024 09:54:30 -0700, Breno Leitao <[email-masked]> wrote:
> >
> > > > 4) Since the command above does not have a key, then the last
> > > >    scatter-gatter entry will be zeroed, since rss_key_size == 0.
> > > >     sg_buf_size = vi->rss_key_size;
> > >
> > >
> > >
> > > 	if (vi->has_rss || vi->has_rss_hash_report) {
> > > 		vi->rss_indir_table_size =
> > > 			virtio_cread16(vdev, offsetof(struct virtio_net_config,
> > > 				rss_max_indirection_table_length));
> > > 		vi->rss_key_size =
> > > 			virtio_cread8(vdev, offsetof(struct virtio_net_config, rss_max_key_size));
> > >
> > > 		vi->rss_hash_types_supported =
> > > 		    virtio_cread32(vdev, offsetof(struct virtio_net_config, supported_hash_types));
> > > 		vi->rss_hash_types_supported &=
> > > 				~(VIRTIO_NET_RSS_HASH_TYPE_IP_EX |
> > > 				  VIRTIO_NET_RSS_HASH_TYPE_TCP_EX |
> > > 				  VIRTIO_NET_RSS_HASH_TYPE_UDP_EX);
> > >
> > > 		dev->hw_features |= NETIF_F_RXHASH;
> > > 	}
> > >
> > >
> > > vi->rss_key_size is initiated here, I wonder if there is something wrong?
> >
> > Not really, the code above is never executed (in my machines). This is
> > because `vi->has_rss` and `vi->has_rss_hash_report` are both unset.
> >
> > Looking further, vdev does not have the VIRTIO_NET_F_RSS and
> > VIRTIO_NET_F_HASH_REPORT features.
> >
> > Also, when I run `ethtool -x`, I got:
> >
> > 	# ethtool  -x eth0
> > 	RX flow hash indirection table for eth0 with 1 RX ring(s):
> > 	Operation not supported
> > 	RSS hash key:
> > 	Operation not supported
> > 	RSS hash function:
> > 	    toeplitz: on
> > 	    xor: off
> > 	    crc32: off
> 
> 
> The spec saies:
> 	Note that if the device offers VIRTIO_NET_F_HASH_REPORT, even if it
> 	supports only one pair of virtqueues, it MUST support at least one of
> 	commands of VIRTIO_NET_CTRL_MQ class to configure reported hash
> 	parameters:
> 
> 	If the device offers VIRTIO_NET_F_RSS, it MUST support
> 	VIRTIO_NET_CTRL_MQ_RSS_CONFIG command per 5.1.6.5.7.1.
> 
> 	Otherwise the device MUST support VIRTIO_NET_CTRL_MQ_HASH_CONFIG command
> 	per 5.1.6.5.6.4.
> 
> 
> So if we have not anyone of `vi->has_rss` and `vi->has_rss_hash_report`,
> we should return from virtnet_set_rxfh directly.

Makes sense. Although it is not clear to me how vi->has_rss_hash_report
is related here, but, I am convinced that we shouldn't do any RSS
operation if the device doesn't have the RSS feature, i.e, vi->has_rss
is false.

That said, I am thinking about something like this. How does it sound?

	diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
	index 5a7700b103f8..8c1ad7361cf2 100644
	--- a/drivers/net/virtio_net.c
	+++ b/drivers/net/virtio_net.c
	@@ -3780,6 +3780,9 @@ static int virtnet_set_rxfh(struct net_device *dev,
		struct virtnet_info *vi = netdev_priv(dev);
		int i;
	 
	+	if (!vi->has_rss)
	+		return -EOPNOTSUPP;
	+
		if (rxfh->hfunc != ETH_RSS_HASH_NO_CHANGE &&
		    rxfh->hfunc != ETH_RSS_HASH_TOP)
			return -EOPNOTSUPP;

Thanks!