netdev - Re: [PATCH] net: Validate frames going through the direct

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Date:	Tue, 02 Sep 2014 16:30:48 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH] net: Validate frames going through the direct_xmit path

On Tue, 2014-09-02 at 18:55 -0400, Alexander Duyck wrote:
> In commit 50cbe9ab5f8d92d2d4a327b56e96559d8f63a1fa "net: Validate xmit SKBs
> right when we pull them out of the qdisc" the validation code was moved out
> of dev_hard_start_xmit and into dequeue_skb.  However this overlooked the
> fact that we do not always enqueue the skb onto a qdisc.
> 
> As a result I was seeing issues trying to connect to a vhost_net interface
> after this patch was applied.  To resolve the issue I have added a call to
> validate_xmit_skb in sched_direct_xmit and this seems to have resolved the
> issue by restoring the validation to this xmit path.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> ---
>  net/sched/sch_generic.c |    9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index a8bf9f9..203ee65 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -128,8 +128,13 @@ int sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
>  	spin_unlock(root_lock);
>  
>  	HARD_TX_LOCK(dev, txq, smp_processor_id());
> -	if (!netif_xmit_frozen_or_stopped(txq))
> -		skb = dev_hard_start_xmit(skb, dev, txq, &ret);
> +	if (!netif_xmit_frozen_or_stopped(txq)) {
> +		skb = validate_xmit_skb(skb, dev);
> +		if (!skb)
> +			ret = NETDEV_TX_OK;
> +		else
> +			skb = dev_hard_start_xmit(skb, dev, txq, &ret);
> +	}
>  
>  	HARD_TX_UNLOCK(dev, txq);
>  

This looks very weird.

Calling validate_xmit_skb() twice per packet is not needed in the case
sch_direct_xmit() is called from qdisc_restart()

This will add bad branch prediction at very minimum.

This is a TCQ_F_CAN_BYPASS issue that should be fixed there.



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