11787 Kernel needs to be built with retpolines
11788 Kernel needs to generally use RSB stuffing
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: John Levon <john.levon@joyent.com>

*** 169,178 ****
--- 169,186 ----
          andq    $PS_ACHK, %rsi
          movq    %rsi, T_USERACC(%rax)
          call    smap_enable
  
          /*
+          * Take a moment to potentially clear the RSB buffer. This is done to
+          * prevent various Spectre variant 2 and SpectreRSB attacks. This may
+          * not be sufficient. Please see uts/intel/ia32/ml/retpoline.s for more
+          * information about this.
+          */
+         call    x86_rsb_stuff
+ 
+         /*
           * Save non-volatile registers, and set return address for current
           * thread to resume_return.
           *
           * %r12 = t (new thread) when done
           */
*** 488,496 ****
          ENTRY(thread_start)
          popq    %rax            /* start() */
          popq    %rdi            /* arg */
          popq    %rsi            /* len */
          movq    %rsp, %rbp
!         call    *%rax
          call    thread_exit     /* destroy thread if it returns. */
          /*NOTREACHED*/
          SET_SIZE(thread_start)
--- 496,504 ----
          ENTRY(thread_start)
          popq    %rax            /* start() */
          popq    %rdi            /* arg */
          popq    %rsi            /* len */
          movq    %rsp, %rbp
!         INDIRECT_CALL_REG(rax)
          call    thread_exit     /* destroy thread if it returns. */
          /*NOTREACHED*/
          SET_SIZE(thread_start)