[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/crypto/x509.c

 }
 
 /** X.509 cache discarder */
-struct cache_discarder x509_cache_discarder __cache_discarder = {
+struct cache_discarder x509_discarder __cache_discarder ( CACHE_NORMAL ) = {
 	.discard = x509_discard,
 };
 

src/include/ipxe/malloc.h

 #define CACHE_DISCARDERS __table ( struct cache_discarder, "cache_discarders" )
 
 /** Declare a cache discarder */
-#define __cache_discarder __table_entry ( CACHE_DISCARDERS, 01 )
+#define __cache_discarder( cost ) __table_entry ( CACHE_DISCARDERS, cost )
+
+/** @defgroup cache_cost Cache discarder costs
+ *
+ * @{
+ */
+
+#define CACHE_CHEAP	01	/**< Items with a low replacement cost */
+#define CACHE_NORMAL	02	/**< Items with a normal replacement cost */
+#define CACHE_EXPENSIVE	03	/**< Items with a high replacement cost */
+
+/** @} */
 
 #endif /* _IPXE_MALLOC_H */

src/net/arp.c

 	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,
 };

src/net/tcp.c

 }
 
 /** TCP cache discarder */
-struct cache_discarder tcp_cache_discarder __cache_discarder = {
+struct cache_discarder tcp_discarder __cache_discarder ( CACHE_NORMAL ) = {
 	.discard = tcp_discard,
 };