13902 Fix for 13717 may break 8-disk raidz2
13915 installctx() blocking allocate causes problems
Portions contributed by: Jerry Jelinek <gjelinek@gmail.com>
Change-Id: I934d69946cec42630fc541fa8c7385b862b69ca2

*** 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 2021 Joyent, Inc.
   */
  
  /*      Copyright (c) 1990, 1991 UNIX System Laboratories, Inc. */
  /*      Copyright (c) 1984, 1986, 1987, 1988, 1989, 1990 AT&T   */
  /*        All Rights Reserved   */
*** 91,100 ****
--- 91,103 ----
           * The SI86V86 subsystem call of the SYSI86 system call
           * supports only one subcode -- V86SC_IOPL.
           */
          case SI86V86:
                  if (arg1 == V86SC_IOPL) {
+ #if defined(__xpv)
+                         struct ctxop *ctx;
+ #endif
                          struct regs *rp = lwptoregs(ttolwp(curthread));
                          greg_t oldpl = rp->r_ps & PS_IOPL;
                          greg_t newpl = arg2 & PS_IOPL;
  
                          /*
*** 103,116 ****
                           */
                          if (newpl > oldpl && (error =
                              secpolicy_sys_config(CRED(), B_FALSE)) != 0)
                                  return (set_errno(error));
  #if defined(__xpv)
                          kpreempt_disable();
                          installctx(curthread, NULL, xen_disable_user_iopl,
                              xen_enable_user_iopl, NULL, NULL,
!                             xen_disable_user_iopl, NULL);
                          xen_enable_user_iopl();
                          kpreempt_enable();
  #else
                          rp->r_ps ^= oldpl ^ newpl;
  #endif
--- 106,120 ----
                           */
                          if (newpl > oldpl && (error =
                              secpolicy_sys_config(CRED(), B_FALSE)) != 0)
                                  return (set_errno(error));
  #if defined(__xpv)
+                         ctx = installctx_preallocate();
                          kpreempt_disable();
                          installctx(curthread, NULL, xen_disable_user_iopl,
                              xen_enable_user_iopl, NULL, NULL,
!                             xen_disable_user_iopl, NULL, ctx);
                          xen_enable_user_iopl();
                          kpreempt_enable();
  #else
                          rp->r_ps ^= oldpl ^ newpl;
  #endif