8956 Implement KPTI
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>

*** 20,29 ****
--- 20,30 ----
   */
  
  /*
   * Copyright 2008 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
+  * Copyright 2018 Joyent, Inc.
   */
  
  
  #if defined(__lint)
  
*** 50,59 ****
--- 51,63 ----
      movb        val, %al;       \
      outb        $0x80;
  
  
  #define DISABLE_PAGING                                                  \
+         movl    %cr4, %eax                                              ;\
+         btrl    $17, %eax       /* clear PCIDE bit */                   ;\
+         movl    %eax, %cr4                                              ;\
          movl    %cr0, %eax                                              ;\
          btrl    $31, %eax       /* clear PG bit */                      ;\
          movl    %eax, %cr0
  
  /*
*** 220,229 ****
--- 224,234 ----
  
          /*
           * Disable long mode by:
           * - shutting down paging (bit 31 of cr0).  This will flush the
           *   TLBs.
+          * - turning off PCID in cr4
           * - disabling LME (long mode enable) in EFER (extended feature reg)
           */
  #endif
          DISABLE_PAGING          /* clobbers %eax */