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

*** 22,32 ****
   * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
  
  /*
!  * Copyright (c) 2013, Joyent, Inc. All rights reserved.
   */
  
  /*
   * Process switching routines.
   */
--- 22,32 ----
   * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
  
  /*
!  * Copyright (c) 2018 Joyent, Inc.
   */
  
  /*
   * Process switching routines.
   */
*** 233,242 ****
--- 233,244 ----
  
  #else   /* __lint */
  
  #if defined(__amd64)
  
+         .global kpti_enable
+ 
          ENTRY(resume)
          movq    %gs:CPU_THREAD, %rax
          leaq    resume_return(%rip), %r11
  
          /*
*** 360,383 ****
          incq    CPU_STATS_SYS_CPUMIGRATE(%r13)
          movq    %r13, T_CPU(%r12)       /* set new thread's CPU pointer */
  
  .setup_cpu:
          /*
!          * Setup rsp0 (kernel stack) in TSS to curthread's stack.
!          * (Note: Since we don't have saved 'regs' structure for all
!          *        the threads we can't easily determine if we need to
!          *        change rsp0. So, we simply change the rsp0 to bottom 
!          *        of the thread stack and it will work for all cases.)
           *
!          * XX64 - Is this correct?
           */
          movq    CPU_TSS(%r13), %r14
          movq    T_STACK(%r12), %rax
          addq    $REGSIZE+MINFRAME, %rax /* to the bottom of thread stack */
! #if !defined(__xpv)
          movq    %rax, TSS_RSP0(%r14)
  #else
          movl    $KDS_SEL, %edi
          movq    %rax, %rsi
          call    HYPERVISOR_stack_switch
  #endif  /* __xpv */
  
--- 362,395 ----
          incq    CPU_STATS_SYS_CPUMIGRATE(%r13)
          movq    %r13, T_CPU(%r12)       /* set new thread's CPU pointer */
  
  .setup_cpu:
          /*
!          * Setup rsp0 (kernel stack) in TSS to curthread's saved regs
!          * structure.  If this thread doesn't have a regs structure above
!          * the stack -- that is, if lwp_stk_init() was never called for the
!          * thread -- this will set rsp0 to the wrong value, but it's harmless
!          * as it's a kernel thread, and it won't actually attempt to implicitly
!          * use the rsp0 via a privilege change.
           *
!          * Note that when we have KPTI enabled on amd64, we never use this
!          * value at all (since all the interrupts have an IST set).
           */
          movq    CPU_TSS(%r13), %r14
+ #if !defined(__xpv)
+         cmpq    $1, kpti_enable
+         jne     1f
+         leaq    CPU_KPTI_TR_RSP(%r13), %rax
+         jmp     2f
+ 1:
          movq    T_STACK(%r12), %rax
          addq    $REGSIZE+MINFRAME, %rax /* to the bottom of thread stack */
! 2:
          movq    %rax, TSS_RSP0(%r14)
  #else
+         movq    T_STACK(%r12), %rax
+         addq    $REGSIZE+MINFRAME, %rax /* to the bottom of thread stack */
          movl    $KDS_SEL, %edi
          movq    %rax, %rsi
          call    HYPERVISOR_stack_switch
  #endif  /* __xpv */