15254 %ymm registers not restored after signal handler
15367 x86 getfpregs() summons corrupting %xmm ghosts
15333 want x86 /proc xregs support (libc_db, libproc, mdb, etc.)
15336 want libc functions for extended ucontext_t
15334 want ps_lwphandle-specific reg routines
15328 FPU_CW_INIT mistreats reserved bit
15335 i86pc fpu_subr.c isn't really platform-specific
15332 setcontext(2) isn't actually noreturn
15331 need <sys/stdalign.h>
Change-Id: I7060aa86042dfb989f77fc3323c065ea2eafa9ad
Conflicts:
    usr/src/uts/common/fs/proc/prcontrol.c
    usr/src/uts/intel/os/archdep.c
    usr/src/uts/intel/sys/ucontext.h
    usr/src/uts/intel/syscall/getcontext.c

@@ -29,10 +29,14 @@
  *      Copyright (c) 1988 AT&T
  *        All Rights Reserved
  */
 
 /*
+ * Copyright 2023 Oxide Computer Company
+ */
+
+/*
  * Establish the default settings for the floating-point state for a C language
  * program:
  *      rounding mode           -- round to nearest default by OS,
  *      exceptions enabled      -- all masked
  *      sticky bits             -- all clear by default by OS.

@@ -46,34 +50,37 @@
 
 #include        "lint.h"
 #include        <sys/types.h>
 #include        <sys/sysi86.h>  /* for SI86FPHW/SI86FPSTART definitions */
 #include        <sys/fp.h>      /* for FPU_CW_INIT and SSE_MXCSR_INIT */
+#include        <upanic.h>
 
 extern int      __fltrounds();
 
 int     _fp_hw;                 /* default: bss: 0 == no hardware */
 int     _sse_hw;                /* default: bss: 0 == no sse */
 int     __flt_rounds;           /* ANSI rounding mode */
 
+#define UPANIC_MSG      "32-bit FPU init failed!"
+
 void
 __fpstart()
 {
         /*
          * query OS for HW status and ensure the x87 and (optional)
          * SSE control words are (will be) set correctly.
          */
         if ((_sse_hw = sysi86(SI86FPSTART,
             &_fp_hw, FPU_CW_INIT, SSE_MXCSR_INIT)) == -1) {
-                extern void _putcw();
-
                 /*
-                 * (fallback to old syscall on old kernels)
+                 * Because the system only supports a 64-bit kernel, the above
+                 * should always work (we assume it does in 64-bit libc). If it
+                 * should fail, we upanic. Note, because the FPU state is
+                 * unknown, we use a static message below and don't rely upon
+                 * strlen(), but rather constant time lengths.
                  */
-                _sse_hw = 0;
-                (void) sysi86(SI86FPHW, &_fp_hw);
-                _putcw(0x133f);
+                upanic(UPANIC_MSG, sizeof (UPANIC_MSG));
         }
 
         /*
          * At this point the x87 fp environment that has been (or more
          * hopefully, will be) established by the kernel is: