16413 Post-barrier Return Stack Buffer (consider no-eIBRS cases)

*** 9,18 ****
--- 9,19 ----
   * http://www.illumos.org/license/CDDL.
   */
  
  /*
   * Copyright 2019 Joyent, Inc.
+  * Copyright 2024 MNX Cloud, Inc.
   */
  
          .file   "retpoline.s"
  
  /*
*** 117,138 ****
          RETPOLINE_MKJUMP(r13)
          RETPOLINE_MKJUMP(r14)
          RETPOLINE_MKJUMP(r15)
  
          /*
!          * The x86_rsb_stuff function is called from pretty arbitrary
!          * contexts. It's much easier for us to save and restore all the
!          * registers we touch rather than clobber them for callers. You must
!          * preserve this property or the system will panic at best.
           */
!         ENTRY(x86_rsb_stuff)
!         /*
!          * These nops are present so we can patch a ret instruction if we need
!          * to disable RSB stuffing because enhanced IBRS is present or we're
!          * disabling mitigations.
!          */
          nop
          nop
          pushq   %rdi
          pushq   %rax
          movl    $16, %edi
          movq    %rsp, %rax
--- 118,140 ----
          RETPOLINE_MKJUMP(r13)
          RETPOLINE_MKJUMP(r14)
          RETPOLINE_MKJUMP(r15)
  
          /*
!          * The x86_rsb_stuff{,_vmexit} functions can be called from pretty
!          * arbitrary contexts. It's much easier for us to save and restore all
!          * the registers we touch rather than clobber them for callers. You
!          * must preserve this property or the system will panic at best.
!          *
!          * The two entry points are because the need to RSB stuff on Intel
!          * depends greatly on factors that are different in the VMEXIT case,
!          * vs. the other switching cases.  See cpuid.c's cpuid_patch_rsb()
!          * for details.
           */
!         ENTRY(x86_rsb_stuff_vmexit)
          nop
+         ALTENTRY(x86_rsb_stuff)
          nop
          pushq   %rdi
          pushq   %rax
          movl    $16, %edi
          movq    %rsp, %rax
*** 152,161 ****
--- 154,164 ----
          movq    %rax, %rsp
          popq    %rax
          popq    %rdi
          ret
          SET_SIZE(x86_rsb_stuff)
+         SET_SIZE(x86_rsb_stuff_vmexit)
  
  #elif defined(__i386)
  
  /*
   * While the kernel is 64-bit only, dboot is still 32-bit, so there are a