netdev - Re: [PATCH net-next v3 2/4] netdevsim: forward skbs from one connected port to another

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Message-ID: <ecefd186-0324-42af-9123-cfcd10267cdb@davidwei.uk>
Date: Fri, 15 Dec 2023 10:31:42 -0800
From: David Wei <dw@...idwei.uk>
To: Jiri Pirko <jiri@...nulli.us>
Cc: Jakub Kicinski <kuba@...nel.org>, Sabrina Dubroca <sd@...asysnail.net>,
 netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v3 2/4] netdevsim: forward skbs from one
 connected port to another

On 2023-12-15 02:45, Jiri Pirko wrote:
> Thu, Dec 14, 2023 at 10:24:41PM CET, dw@...idwei.uk wrote:
>> Forward skbs sent from one netdevsim port to its connected netdevsim
>> port using dev_forward_skb, in a spirit similar to veth.
> 
> Perhaps better to write "dev_forward_skb()" to make obvious you talk
> about function.

Sorry, it's a bad habit at this point :)

> 
> 
>>
>> Signed-off-by: David Wei <dw@...idwei.uk>
>> ---
>> drivers/net/netdevsim/netdev.c | 23 ++++++++++++++++++-----
>> 1 file changed, 18 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
>> index e290c54b0e70..c5f53b1dbdcc 100644
>> --- a/drivers/net/netdevsim/netdev.c
>> +++ b/drivers/net/netdevsim/netdev.c
>> @@ -29,19 +29,33 @@
>> static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
>> {
>> 	struct netdevsim *ns = netdev_priv(dev);
>> +	struct netdevsim *peer_ns;
>> +	int ret = NETDEV_TX_OK;
>>
>> +	rcu_read_lock();
> 
> Why do you need to be in rcu read locked section here?

So the RCU protected pointer `peer` does not change during the critical
section. Veth does something similar in its xmit() for its peer.

> 
> 
>> 	if (!nsim_ipsec_tx(ns, skb))
>> -		goto out;
>> +		goto err;
> 
> Not sure why you need to rename the label. Why "out" is not okay?
> 
>>
>> 	u64_stats_update_begin(&ns->syncp);
>> 	ns->tx_packets++;
>> 	ns->tx_bytes += skb->len;
>> 	u64_stats_update_end(&ns->syncp);
>>
>> -out:
>> -	dev_kfree_skb(skb);
>> +	peer_ns = rcu_dereference(ns->peer);
>> +	if (!peer_ns)
>> +		goto err;
> 
> This is definitelly not an error path, "err" label name is misleading.

That's fair, I can change it back. Lots has changed since my original
intentions.

> 
> 
>> +
>> +	skb_tx_timestamp(skb);
>> +	if (unlikely(dev_forward_skb(peer_ns->netdev, skb) == NET_RX_DROP))
>> +		ret = NET_XMIT_DROP;
> 
> Hmm, can't you track dropped packets in ns->tx_dropped and expose in
> nsim_get_stats64() ?

I can add this.

> 
> 
>>
>> -	return NETDEV_TX_OK;
>> +	rcu_read_unlock();
>> +	return ret;
>> +
>> +err:
>> +	rcu_read_unlock();
>> +	dev_kfree_skb(skb);
>> +	return ret;
>> }
>>
>> static void nsim_set_rx_mode(struct net_device *dev)
>> @@ -302,7 +316,6 @@ static void nsim_setup(struct net_device *dev)
>> 	eth_hw_addr_random(dev);
>>
>> 	dev->tx_queue_len = 0;
>> -	dev->flags |= IFF_NOARP;
>> 	dev->flags &= ~IFF_MULTICAST;
>> 	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE |
>> 			   IFF_NO_QUEUE;
>> -- 
>> 2.39.3
>>