xen-netback: require fewer guest Rx slots when not using GSO
Commit f48da8b1 (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 1650d545 (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> Reviewed-by: Wei Liu <wei.liu2@citrix.com> Signed-off-by: David S. Miller <davem@davemloft.net>