netdev - Re: [PATCHv1 net] xen-netback: require fewer guest Rx slots when not using GSO

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Date:	Wed, 9 Sep 2015 11:41:20 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	<netdev@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
	Ian Campbell <ian.campbell@...rix.com>,
	Wei Liu <wei.liu2@...rix.com>
Subject: Re: [PATCHv1 net] xen-netback: require fewer guest Rx slots when not
 using GSO

On Tue, Sep 08, 2015 at 02:25:14PM +0100, David Vrabel wrote:
> Commit f48da8b14d04ca87ffcffe68829afd45f926ec6a (xen-netback: fix
> unlimited guest Rx internal queue and carrier flapping) introduced a
> regression.
> 
> The PV frontend in IPXE only places 4 requests on the guest Rx ring.
> Since netback required at least (MAX_SKB_FRAGS + 1) slots, IPXE could
> not receive any packets.
> 
> a) If GSO is not enabled on the VIF, fewer guest Rx slots are required
>    for the largest possible packet.  Calculate the required slots
>    based on the maximum GSO size or the MTU.
> 
>    This calculation of the number of required slots relies on
>    1650d5455bd2 (xen-netback: always fully coalesce guest Rx packets)
>    which present in 4.0-rc1 and later.
> 
> b) Reduce the Rx stall detection to checking for at least one
>    available Rx request.  This is fine since we're predominately
>    concerned with detecting interfaces which are down and thus have
>    zero available Rx requests.
> 
> Signed-off-by: David Vrabel <david.vrabel@...rix.com>

Reviewed-by: Wei Liu <wei.liu2@...rix.com>
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