OS-7125 Need mitigation of L1TF (CVE-2018-3646)
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>

*** 397,406 ****
--- 397,409 ----
   * CPU affinity is guaranteed after return of thread_affinity_set().  If a
   * caller setting affinity to CPU_CURRENT requires that its thread not migrate
   * CPUs prior to a successful return, it should take extra precautions (such as
   * their own call to kpreempt_disable) to ensure that safety.
   *
+  * CPU_BEST can be used to pick a "best" CPU to migrate to, including
+  * potentially the current CPU.
+  *
   * A CPU affinity reference count is maintained by thread_affinity_set and
   * thread_affinity_clear (incrementing and decrementing it, respectively),
   * maintaining CPU affinity while the count is non-zero, and allowing regions
   * of code which require affinity to be nested.
   */
*** 413,422 ****
--- 416,429 ----
  
          if (cpu_id == CPU_CURRENT) {
                  VERIFY3P(t, ==, curthread);
                  kpreempt_disable();
                  cp = CPU;
+         } else if (cpu_id == CPU_BEST) {
+                 VERIFY3P(t, ==, curthread);
+                 kpreempt_disable();
+                 cp = disp_choose_best_cpu();
          } else {
                  /*
                   * We should be asserting that cpu_lock is held here, but
                   * the NCA code doesn't acquire it.  The following assert
                   * should be uncommented when the NCA code is fixed.
*** 450,462 ****
                  force_thread_migrate(t);        /* drops thread lock */
          } else {
                  thread_unlock(t);
          }
  
!         if (cpu_id == CPU_CURRENT) {
                  kpreempt_enable();
-         }
  }
  
  /*
   *      Wrapper for backward compatibility.
   */
--- 457,468 ----
                  force_thread_migrate(t);        /* drops thread lock */
          } else {
                  thread_unlock(t);
          }
  
!         if (cpu_id == CPU_CURRENT || cpu_id == CPU_BEST)
                  kpreempt_enable();
  }
  
  /*
   *      Wrapper for backward compatibility.
   */
*** 1487,1498 ****
  
                                  /*
                                   * Update CPU last ran on if it was this CPU
                                   */
                                  if (t->t_cpu == cp && t->t_bound_cpu != cp)
!                                         t->t_cpu = disp_lowpri_cpu(ncp,
!                                             t->t_lpl, t->t_pri, NULL);
                                  ASSERT(t->t_cpu != cp || t->t_bound_cpu == cp ||
                                      t->t_weakbound_cpu == cp);
  
                                  t = t->t_forw;
                          } while (t != p->p_tlist);
--- 1493,1504 ----
  
                                  /*
                                   * Update CPU last ran on if it was this CPU
                                   */
                                  if (t->t_cpu == cp && t->t_bound_cpu != cp)
!                                         t->t_cpu = disp_lowpri_cpu(ncp, t,
!                                             t->t_pri);
                                  ASSERT(t->t_cpu != cp || t->t_bound_cpu == cp ||
                                      t->t_weakbound_cpu == cp);
  
                                  t = t->t_forw;
                          } while (t != p->p_tlist);
*** 1530,1543 ****
  
                          /*
                           * Update CPU last ran on if it was this CPU
                           */
  
!                         if (t->t_cpu == cp && t->t_bound_cpu != cp) {
!                                 t->t_cpu = disp_lowpri_cpu(ncp,
!                                     t->t_lpl, t->t_pri, NULL);
!                         }
                          ASSERT(t->t_cpu != cp || t->t_bound_cpu == cp ||
                              t->t_weakbound_cpu == cp);
                          t = t->t_next;
  
                  } while (t != curthread);
--- 1536,1548 ----
  
                          /*
                           * Update CPU last ran on if it was this CPU
                           */
  
!                         if (t->t_cpu == cp && t->t_bound_cpu != cp)
!                                 t->t_cpu = disp_lowpri_cpu(ncp, t, t->t_pri);
! 
                          ASSERT(t->t_cpu != cp || t->t_bound_cpu == cp ||
                              t->t_weakbound_cpu == cp);
                          t = t->t_next;
  
                  } while (t != curthread);