11859 need swapgs mitigation
Reviewed by: Robert Mustacchi <rm@fingolfin.org>
Reviewed by: Dan McDonald <danmcd@joyent.com>
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Robert Mustacchi <rm@fingolfin.org>

*** 18,28 ****
   *
   * CDDL HEADER END
   */
  /*
   * Copyright (c) 1992, 2010, Oracle and/or its affiliates. All rights reserved.
!  * Copyright 2018 Joyent, Inc.
   */
  
  /*      Copyright (c) 1990, 1991 UNIX System Laboratories, Inc. */
  /*      Copyright (c) 1984, 1986, 1987, 1988, 1989, 1990 AT&T   */
  /*      All Rights Reserved   */
--- 18,28 ----
   *
   * CDDL HEADER END
   */
  /*
   * Copyright (c) 1992, 2010, Oracle and/or its affiliates. All rights reserved.
!  * Copyright 2019 Joyent, Inc.
   */
  
  /*      Copyright (c) 1990, 1991 UNIX System Laboratories, Inc. */
  /*      Copyright (c) 1984, 1986, 1987, 1988, 1989, 1990 AT&T   */
  /*      All Rights Reserved   */
*** 549,568 ****
                   * we'll be taking a trap but with the original %gs and %gsbase
                   * undamaged (i.e. pointing at curcpu).
                   *
                   * We've just mucked up the kernel's gsbase.  Oops.  In
                   * particular we can't take any traps at all.  Make the newly
!                  * computed gsbase be the hidden gs via __swapgs, and fix
                   * the kernel's gsbase back again. Later, when we return to
                   * userland we'll swapgs again restoring gsbase just loaded
                   * above.
                   */
!                 __swapgs();
                  rp->r_gs = pcb->pcb_gs;
  
                  /*
!                  * restore kernel's gsbase
                   */
                  wrmsr(MSR_AMD_GSBASE, kgsbase);
  
  #endif  /* __xpv */
  
--- 549,571 ----
                   * we'll be taking a trap but with the original %gs and %gsbase
                   * undamaged (i.e. pointing at curcpu).
                   *
                   * We've just mucked up the kernel's gsbase.  Oops.  In
                   * particular we can't take any traps at all.  Make the newly
!                  * computed gsbase be the hidden gs via swapgs, and fix
                   * the kernel's gsbase back again. Later, when we return to
                   * userland we'll swapgs again restoring gsbase just loaded
                   * above.
                   */
!                 __asm__ __volatile__("mfence; swapgs");
! 
                  rp->r_gs = pcb->pcb_gs;
  
                  /*
!                  * Restore kernel's gsbase. Note that this also serializes any
!                  * attempted speculation from loading the user-controlled
!                  * %gsbase.
                   */
                  wrmsr(MSR_AMD_GSBASE, kgsbase);
  
  #endif  /* __xpv */