XXX Nemo stack overflow prevention

*** 20,36 ****
--- 20,41 ----
   */
  /*
   * Copyright 2010 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
+ /*
+  * Copyright 2011 Joyent, Inc.  All rights reserved.
+  */
  
  #include <sys/types.h>
  #include <sys/callb.h>
  #include <sys/sdt.h>
  #include <sys/strsubr.h>
  #include <sys/strsun.h>
  #include <sys/vlan.h>
+ #include <sys/stack.h>
+ #include <sys/archsystm.h>
  #include <inet/ipsec_impl.h>
  #include <inet/ip_impl.h>
  #include <inet/sadb.h>
  #include <inet/ipsecesp.h>
  #include <inet/ipsecah.h>
*** 413,422 ****
--- 418,449 ----
          cookie = (mac_tx_cookie_t)srs;                                  \
          *ret_mp = mp_chain;                                             \
  }
  
  /*
+  * MAC_RX_SRS_TOODEEP
+  *
+  * Macro called as part of receive-side processing to determine if handling
+  * can occur in situ (in the interrupt thread) or if it should be left to a
+  * worker thread.  Note that the constant used to make this determination is
+  * not entirely made-up, and is a result of some emprical validation. That
+  * said, the constant is left as a static variable to allow it to be
+  * dynamically tuned in the field if and as needed.
+  */
+ static uintptr_t mac_rx_srs_stack_needed = 8192;
+ static uint_t mac_rx_srs_stack_toodeep;
+ 
+ #ifndef STACK_GROWTH_DOWN
+ #error Downward stack growth assumed.
+ #endif
+ 
+ #define MAC_RX_SRS_TOODEEP() (STACK_BIAS + (uintptr_t)getfp() - \
+         (uintptr_t)curthread->t_stkbase < mac_rx_srs_stack_needed && \
+         ++mac_rx_srs_stack_toodeep)
+ 
+ 
+ /*
   * Drop the rx packet and advance to the next one in the chain.
   */
  static void
  mac_rx_drop_pkt(mac_soft_ring_set_t *srs, mblk_t *mp)
  {
*** 2417,2431 ****
  
          MAC_RX_SRS_ENQUEUE_CHAIN(mac_srs, mp_chain, tail, count, sz);
  
          if (!(mac_srs->srs_state & SRS_PROC)) {
                  /*
!                  * If we are coming via loopback or if we are not
!                  * optimizing for latency, we should signal the
!                  * worker thread.
                   */
!                 if (loopback || !(mac_srs->srs_state & SRS_LATENCY_OPT)) {
                          /*
                           * For loopback, We need to let the worker take
                           * over as we don't want to continue in the same
                           * thread even if we can. This could lead to stack
                           * overflows and may also end up using
--- 2444,2459 ----
  
          MAC_RX_SRS_ENQUEUE_CHAIN(mac_srs, mp_chain, tail, count, sz);
  
          if (!(mac_srs->srs_state & SRS_PROC)) {
                  /*
!                  * If we are coming via loopback, if we are not optimizing for
!                  * latency, or if our stack is running deep, we should signal
!                  * the worker thread.
                   */
!                 if (loopback || !(mac_srs->srs_state & SRS_LATENCY_OPT) ||
!                     MAC_RX_SRS_TOODEEP()) {
                          /*
                           * For loopback, We need to let the worker take
                           * over as we don't want to continue in the same
                           * thread even if we can. This could lead to stack
                           * overflows and may also end up using