netdev - Re: [PATCH net-next 0/2] xen-netback: Changes around carrier handling

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Date:	Thu, 07 Aug 2014 22:28:01 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	zoltan.kiss@...rix.com
Cc:	wei.liu2@...rix.com, Ian.Campbell@...rix.com,
	david.vrabel@...rix.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	Paul.Durrant@...rix.com
Subject: Re: [PATCH net-next 0/2] xen-netback: Changes around carrier
 handling

From: Zoltan Kiss <zoltan.kiss@...rix.com>
Date: Thu, 7 Aug 2014 16:51:17 +0100

> Ok, how about this:
> * ndo_start_xmit tries to set up the grant copy operations, something
> * which is done now in the thread
> * no estimation madness, just go ahead and try to do it
> * if the skb can fit, kick the thread
> * if it fails (not enough slots to complete the TX), then:
>   * call netif_tx_stop_queue on that queue (just like now)
>   * set up timer rx_stalled (just like now)
>   * save the state of the current skb (where the grant copy op setup is
>   * halted)
> * if new slots coming in, continue to create the grant copy ops for the
> * stalled skb, and if it succeeds, kick the thread plus call
> * netif_tx_start_queue. (just like now)
> * if the timer fires, drop the stalled skb, and set the carrier off, so
> * QDisc won't bother to queue packets for a stalled interface
> * the thread will only do the actual grant copy hypercall and releasing
> * the skb
> * in any case, ndo_start_xmit should return NETDEV_TX_OK, just like now

It sounds like this would work, and indeed it would abide by the intended
rules of netif_{stop,wake}_queue() and ->ndo_start_xmit()'s return
values.
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