copy_pending_idx
pending_idx = copy_pending_idx(skb, 0);
u16 copy_pending_idx[XEN_NETBK_LEGACY_SLOTS_MAX + 1];
#define copy_pending_idx(skb, i) (XENVIF_TX_CB(skb)->copy_pending_idx[i])
copy_pending_idx(skb, copy_count(skb)) = pending_idx;
copy_pending_idx(skb, copy_count(skb) - 1);
pending_idx = copy_pending_idx(skb, i);