[arp] Try to avoid discarding ARP cache entries

Discarding the active ARP cache entry in the middle of a download will
substantially disrupt the TCP stream.  Try to minimise any such
disruption by treating ARP cache entries as expensive, and discarding
them only when nothing else is available to discard.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/arp.c

@@ -525,7 +525,12 @@ static unsigned int arp_discard ( void ) {
 	return 0;
 }
 
-/** ARP cache discarder */
-struct cache_discarder arp_cache_discarder __cache_discarder = {
+/** ARP cache discarder
+ *
+ * ARP cache entries are deemed to have a high replacement cost, since
+ * flushing an active ARP cache entry midway through a TCP transfer
+ * will cause substantial disruption.
+ */
+struct cache_discarder arp_discarder __cache_discarder ( CACHE_EXPENSIVE ) = {
 	.discard = arp_discard,
 };