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

[David Miller <davem@davemloft.net>]

From: David Vrabel <david.vrabel@citrix.com>
Date: Tue, 8 Sep 2015 14:25:14 +0100

> 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@citrix.com>

Applied, thanks David.