sys/platform/pc32/i386/swtch.s

   1 /*
   2  * Copyright (c) 2003,2004 The DragonFly Project.  All rights reserved.
   3  *
   4  * This code is derived from software contributed to The DragonFly Project
   5  * by Matthew Dillon <dillon@backplane.com>
   6  *
   7  * Redistribution and use in source and binary forms, with or without
   8  * modification, are permitted provided that the following conditions
   9  * are met:
  10  *
  11  * 1. Redistributions of source code must retain the above copyright
  12  *    notice, this list of conditions and the following disclaimer.
  13  * 2. Redistributions in binary form must reproduce the above copyright
  14  *    notice, this list of conditions and the following disclaimer in
  15  *    the documentation and/or other materials provided with the
  16  *    distribution.
  17  * 3. Neither the name of The DragonFly Project nor the names of its
  18  *    contributors may be used to endorse or promote products derived
  19  *    from this software without specific, prior written permission.
  20  *
  21  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  22  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  23  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  24  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
  25  * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  26  * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
  27  * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  28  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
  29  * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  30  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
  31  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  32  * SUCH DAMAGE.
  33  *
  34  * Copyright (c) 1990 The Regents of the University of California.
  35  * All rights reserved.
  36  *
  37  * This code is derived from software contributed to Berkeley by
  38  * William Jolitz.
  39  *
  40  * Redistribution and use in source and binary forms, with or without
  41  * modification, are permitted provided that the following conditions
  42  * are met:
  43  * 1. Redistributions of source code must retain the above copyright
  44  *    notice, this list of conditions and the following disclaimer.
  45  * 2. Redistributions in binary form must reproduce the above copyright
  46  *    notice, this list of conditions and the following disclaimer in the
  47  *    documentation and/or other materials provided with the distribution.
  48  * 3. All advertising materials mentioning features or use of this software
  49  *    must display the following acknowledgement:
  50  *      This product includes software developed by the University of
  51  *      California, Berkeley and its contributors.
  52  * 4. Neither the name of the University nor the names of its contributors
  53  *    may be used to endorse or promote products derived from this software
  54  *    without specific prior written permission.
  55  *
  56  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  57  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  58  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  59  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  60  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  61  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  62  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  63  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  64  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  65  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  66  * SUCH DAMAGE.
  67  *
  68  * $FreeBSD: src/sys/i386/i386/swtch.s,v 1.89.2.10 2003/01/23 03:36:24 ps Exp $
  69  * $DragonFly: src/sys/platform/pc32/i386/swtch.s,v 1.47 2007/06/29 21:54:10 dillon Exp $
  70  */
  71
  72 #include "use_npx.h"
  73
  74 #include <sys/rtprio.h>
  75
  76 #include <machine/asmacros.h>
  77 #include <machine/segments.h>
  78
  79 #include <machine/pmap.h>
  80 #include <machine_base/apic/apicreg.h>
  81 #include <machine/lock.h>
  82
  83 #include "assym.s"
  84
  85 #if defined(SMP)
  86 #define MPLOCKED        lock ;
  87 #else
  88 #define MPLOCKED
  89 #endif
  90
  91         .data
  92
  93         .globl  panic
  94
  95 #if defined(SWTCH_OPTIM_STATS)
  96         .globl  swtch_optim_stats, tlb_flush_count
  97 swtch_optim_stats:      .long   0               /* number of _swtch_optims */
  98 tlb_flush_count:        .long   0
  99 #endif
 100
 101         .text
 102
 103
 104 /*
 105  * cpu_heavy_switch(next_thread)
 106  *
 107  *      Switch from the current thread to a new thread.  This entry
 108  *      is normally called via the thread->td_switch function, and will
 109  *      only be called when the current thread is a heavy weight process.
 110  *
 111  *      Some instructions have been reordered to reduce pipeline stalls.
 112  *
 113  *      YYY disable interrupts once giant is removed.
 114  */
 115 ENTRY(cpu_heavy_switch)
 116         /*
 117          * Save general regs
 118          */
 119         movl    PCPU(curthread),%ecx
 120         movl    (%esp),%eax                     /* (reorder optimization) */
 121         movl    TD_PCB(%ecx),%edx               /* EDX = PCB */
 122         movl    %eax,PCB_EIP(%edx)              /* return PC may be modified */
 123         movl    %ebx,PCB_EBX(%edx)
 124         movl    %esp,PCB_ESP(%edx)
 125         movl    %ebp,PCB_EBP(%edx)
 126         movl    %esi,PCB_ESI(%edx)
 127         movl    %edi,PCB_EDI(%edx)
 128
 129         movl    %ecx,%ebx                       /* EBX = curthread */
 130         movl    TD_LWP(%ecx),%ecx
 131         movl    PCPU(cpuid), %eax
 132         movl    LWP_VMSPACE(%ecx), %ecx         /* ECX = vmspace */
 133         MPLOCKED btrl   %eax, VM_PMAP+PM_ACTIVE(%ecx)
 134
 135         /*
 136          * Push the LWKT switch restore function, which resumes a heavy
 137          * weight process.  Note that the LWKT switcher is based on
 138          * TD_SP, while the heavy weight process switcher is based on
 139          * PCB_ESP.  TD_SP is usually two ints pushed relative to
 140          * PCB_ESP.  We push the flags for later restore by cpu_heavy_restore.
 141          */
 142         pushfl
 143         pushl   $cpu_heavy_restore
 144         movl    %esp,TD_SP(%ebx)
 145
 146         /*
 147          * Save debug regs if necessary
 148          */
 149         movb    PCB_FLAGS(%edx),%al
 150         andb    $PCB_DBREGS,%al
 151         jz      1f                              /* no, skip over */
 152         movl    %dr7,%eax                       /* yes, do the save */
 153         movl    %eax,PCB_DR7(%edx)
 154         andl    $0x0000fc00, %eax               /* disable all watchpoints */
 155         movl    %eax,%dr7
 156         movl    %dr6,%eax
 157         movl    %eax,PCB_DR6(%edx)
 158         movl    %dr3,%eax
 159         movl    %eax,PCB_DR3(%edx)
 160         movl    %dr2,%eax
 161         movl    %eax,PCB_DR2(%edx)
 162         movl    %dr1,%eax
 163         movl    %eax,PCB_DR1(%edx)
 164         movl    %dr0,%eax
 165         movl    %eax,PCB_DR0(%edx)
 166 1:
 167
 168 #if NNPX > 0
 169         /*
 170          * Save the FP state if we have used the FP.  Note that calling
 171          * npxsave will NULL out PCPU(npxthread).
 172          */
 173         cmpl    %ebx,PCPU(npxthread)
 174         jne     1f
 175         pushl   TD_SAVEFPU(%ebx)
 176         call    npxsave                 /* do it in a big C function */
 177         addl    $4,%esp                 /* EAX, ECX, EDX trashed */
 178 1:
 179 #endif  /* NNPX > 0 */
 180
 181         /*
 182          * Switch to the next thread, which was passed as an argument
 183          * to cpu_heavy_switch().  Due to the eflags and switch-restore
 184          * function we pushed, the argument is at 12(%esp).  Set the current
 185          * thread, load the stack pointer, and 'ret' into the switch-restore
 186          * function.
 187          *
 188          * The switch restore function expects the new thread to be in %eax
 189          * and the old one to be in %ebx.
 190          *
 191          * There is a one-instruction window where curthread is the new
 192          * thread but %esp still points to the old thread's stack, but
 193          * we are protected by a critical section so it is ok.
 194          */
 195         movl    12(%esp),%eax           /* EAX = newtd, EBX = oldtd */
 196         movl    %eax,PCPU(curthread)
 197         movl    TD_SP(%eax),%esp
 198         ret
 199
 200 /*
 201  *  cpu_exit_switch()
 202  *
 203  *      The switch function is changed to this when a thread is going away
 204  *      for good.  We have to ensure that the MMU state is not cached, and
 205  *      we don't bother saving the existing thread state before switching.
 206  *
 207  *      At this point we are in a critical section and this cpu owns the
 208  *      thread's token, which serves as an interlock until the switchout is
 209  *      complete.
 210  */
 211 ENTRY(cpu_exit_switch)
 212         /*
 213          * Get us out of the vmspace
 214          */
 215         movl    IdlePTD,%ecx
 216         movl    %cr3,%eax
 217         cmpl    %ecx,%eax
 218         je      1f
 219         movl    %ecx,%cr3
 220 1:
 221         movl    PCPU(curthread),%ebx
 222
 223         /*
 224          * If this is a process/lwp, deactivate the pmap after we've
 225          * switched it out.
 226          */
 227         movl    TD_LWP(%ebx),%ecx
 228         testl   %ecx,%ecx
 229         jz      2f
 230         movl    PCPU(cpuid), %eax
 231         movl    LWP_VMSPACE(%ecx), %ecx         /* ECX = vmspace */
 232         MPLOCKED btrl   %eax, VM_PMAP+PM_ACTIVE(%ecx)
 233 2:
 234         /*
 235          * Switch to the next thread.  RET into the restore function, which
 236          * expects the new thread in EAX and the old in EBX.
 237          *
 238          * There is a one-instruction window where curthread is the new
 239          * thread but %esp still points to the old thread's stack, but
 240          * we are protected by a critical section so it is ok.
 241          */
 242         movl    4(%esp),%eax
 243         movl    %eax,PCPU(curthread)
 244         movl    TD_SP(%eax),%esp
 245         ret
 246
 247 /*
 248  * cpu_heavy_restore()  (current thread in %eax on entry)
 249  *
 250  *      Restore the thread after an LWKT switch.  This entry is normally
 251  *      called via the LWKT switch restore function, which was pulled
 252  *      off the thread stack and jumped to.
 253  *
 254  *      This entry is only called if the thread was previously saved
 255  *      using cpu_heavy_switch() (the heavy weight process thread switcher),
 256  *      or when a new process is initially scheduled.  The first thing we
 257  *      do is clear the TDF_RUNNING bit in the old thread and set it in the
 258  *      new thread.
 259  *
 260  *      NOTE: The lwp may be in any state, not necessarily LSRUN, because
 261  *      a preemption switch may interrupt the process and then return via
 262  *      cpu_heavy_restore.
 263  *
 264  *      YYY theoretically we do not have to restore everything here, a lot
 265  *      of this junk can wait until we return to usermode.  But for now
 266  *      we restore everything.
 267  *
 268  *      YYY the PCB crap is really crap, it makes startup a bitch because
 269  *      we can't switch away.
 270  *
 271  *      YYY note: spl check is done in mi_switch when it splx()'s.
 272  */
 273
 274 ENTRY(cpu_heavy_restore)
 275         popfl
 276         movl    TD_PCB(%eax),%edx               /* EDX = PCB */
 277         movl    TD_LWP(%eax),%ecx
 278
 279 #if defined(SWTCH_OPTIM_STATS)
 280         incl    _swtch_optim_stats
 281 #endif
 282         /*
 283          * Tell the pmap that our cpu is using the VMSPACE now.  We cannot
 284          * safely test/reload %cr3 until after we have set the bit in the
 285          * pmap (remember, we do not hold the MP lock in the switch code).
 286          */
 287         movl    LWP_VMSPACE(%ecx), %ecx         /* ECX = vmspace */
 288         movl    PCPU(cpuid), %esi
 289         MPLOCKED btsl   %esi, VM_PMAP+PM_ACTIVE(%ecx)
 290
 291         /*
 292          * Restore the MMU address space.  If it is the same as the last
 293          * thread we don't have to invalidate the tlb (i.e. reload cr3).
 294          * YYY which naturally also means that the PM_ACTIVE bit had better
 295          * already have been set before we set it above, check? YYY
 296          */
 297         movl    %cr3,%esi
 298         movl    PCB_CR3(%edx),%ecx
 299         cmpl    %esi,%ecx
 300         je      4f
 301 #if defined(SWTCH_OPTIM_STATS)
 302         decl    _swtch_optim_stats
 303         incl    _tlb_flush_count
 304 #endif
 305         movl    %ecx,%cr3
 306 4:
 307         /*
 308          * Clear TDF_RUNNING flag in old thread only after cleaning up
 309          * %cr3.  The target thread is already protected by being TDF_RUNQ
 310          * so setting TDF_RUNNING isn't as big a deal.
 311          */
 312         andl    $~TDF_RUNNING,TD_FLAGS(%ebx)
 313         orl     $TDF_RUNNING,TD_FLAGS(%eax)
 314
 315         /*
 316          * Deal with the PCB extension, restore the private tss
 317          */
 318         movl    PCB_EXT(%edx),%edi      /* check for a PCB extension */
 319         movl    $1,%ebx                 /* maybe mark use of a private tss */
 320         testl   %edi,%edi
 321         jnz     2f
 322
 323         /*
 324          * Going back to the common_tss.  We may need to update TSS_ESP0
 325          * which sets the top of the supervisor stack when entering from
 326          * usermode.  The PCB is at the top of the stack but we need another
 327          * 16 bytes to take vm86 into account.
 328          */
 329         leal    -16(%edx),%ebx
 330         movl    %ebx, PCPU(common_tss) + TSS_ESP0
 331
 332         cmpl    $0,PCPU(private_tss)    /* don't have to reload if      */
 333         je      3f                      /* already using the common TSS */
 334
 335         subl    %ebx,%ebx               /* unmark use of private tss */
 336
 337         /*
 338          * Get the address of the common TSS descriptor for the ltr.
 339          * There is no way to get the address of a segment-accessed variable
 340          * so we store a self-referential pointer at the base of the per-cpu
 341          * data area and add the appropriate offset.
 342          */
 343         movl    $gd_common_tssd, %edi
 344         addl    %fs:0, %edi
 345
 346         /*
 347          * Move the correct TSS descriptor into the GDT slot, then reload
 348          * ltr.
 349          */
 350 2:
 351         movl    %ebx,PCPU(private_tss)          /* mark/unmark private tss */
 352         movl    PCPU(tss_gdt), %ebx             /* entry in GDT */
 353         movl    0(%edi), %eax
 354         movl    %eax, 0(%ebx)
 355         movl    4(%edi), %eax
 356         movl    %eax, 4(%ebx)
 357         movl    $GPROC0_SEL*8, %esi             /* GSEL(entry, SEL_KPL) */
 358         ltr     %si
 359
 360 3:
 361         /*
 362          * Restore general registers.
 363          */
 364         movl    PCB_EBX(%edx),%ebx
 365         movl    PCB_ESP(%edx),%esp
 366         movl    PCB_EBP(%edx),%ebp
 367         movl    PCB_ESI(%edx),%esi
 368         movl    PCB_EDI(%edx),%edi
 369         movl    PCB_EIP(%edx),%eax
 370         movl    %eax,(%esp)
 371
 372         /*
 373          * Restore the user LDT if we have one
 374          */
 375         cmpl    $0, PCB_USERLDT(%edx)
 376         jnz     1f
 377         movl    _default_ldt,%eax
 378         cmpl    PCPU(currentldt),%eax
 379         je      2f
 380         lldt    _default_ldt
 381         movl    %eax,PCPU(currentldt)
 382         jmp     2f
 383 1:      pushl   %edx
 384         call    set_user_ldt
 385         popl    %edx
 386 2:
 387         /*
 388          * Restore the user TLS if we have one
 389          */
 390         pushl   %edx
 391         call    set_user_TLS
 392         popl    %edx
 393
 394         /*
 395          * Restore the DEBUG register state if necessary.
 396          */
 397         movb    PCB_FLAGS(%edx),%al
 398         andb    $PCB_DBREGS,%al
 399         jz      1f                              /* no, skip over */
 400         movl    PCB_DR6(%edx),%eax              /* yes, do the restore */
 401         movl    %eax,%dr6
 402         movl    PCB_DR3(%edx),%eax
 403         movl    %eax,%dr3
 404         movl    PCB_DR2(%edx),%eax
 405         movl    %eax,%dr2
 406         movl    PCB_DR1(%edx),%eax
 407         movl    %eax,%dr1
 408         movl    PCB_DR0(%edx),%eax
 409         movl    %eax,%dr0
 410         movl    %dr7,%eax                /* load dr7 so as not to disturb */
 411         andl    $0x0000fc00,%eax         /*   reserved bits               */
 412         pushl   %ebx
 413         movl    PCB_DR7(%edx),%ebx
 414         andl    $~0x0000fc00,%ebx
 415         orl     %ebx,%eax
 416         popl    %ebx
 417         movl    %eax,%dr7
 418 1:
 419
 420         ret
 421
 422 /*
 423  * savectx(pcb)
 424  *
 425  * Update pcb, saving current processor state.
 426  */
 427 ENTRY(savectx)
 428         /* fetch PCB */
 429         movl    4(%esp),%ecx
 430
 431         /* caller's return address - child won't execute this routine */
 432         movl    (%esp),%eax
 433         movl    %eax,PCB_EIP(%ecx)
 434
 435         movl    %cr3,%eax
 436         movl    %eax,PCB_CR3(%ecx)
 437
 438         movl    %ebx,PCB_EBX(%ecx)
 439         movl    %esp,PCB_ESP(%ecx)
 440         movl    %ebp,PCB_EBP(%ecx)
 441         movl    %esi,PCB_ESI(%ecx)
 442         movl    %edi,PCB_EDI(%ecx)
 443
 444 #if NNPX > 0
 445         /*
 446          * If npxthread == NULL, then the npx h/w state is irrelevant and the
 447          * state had better already be in the pcb.  This is true for forks
 448          * but not for dumps (the old book-keeping with FP flags in the pcb
 449          * always lost for dumps because the dump pcb has 0 flags).
 450          *
 451          * If npxthread != NULL, then we have to save the npx h/w state to
 452          * npxthread's pcb and copy it to the requested pcb, or save to the
 453          * requested pcb and reload.  Copying is easier because we would
 454          * have to handle h/w bugs for reloading.  We used to lose the
 455          * parent's npx state for forks by forgetting to reload.
 456          */
 457         movl    PCPU(npxthread),%eax
 458         testl   %eax,%eax
 459         je      1f
 460
 461         pushl   %ecx                    /* target pcb */
 462         movl    TD_SAVEFPU(%eax),%eax   /* originating savefpu area */
 463         pushl   %eax
 464
 465         pushl   %eax
 466         call    npxsave
 467         addl    $4,%esp
 468
 469         popl    %eax
 470         popl    %ecx
 471
 472         pushl   $PCB_SAVEFPU_SIZE
 473         leal    PCB_SAVEFPU(%ecx),%ecx
 474         pushl   %ecx
 475         pushl   %eax
 476         call    bcopy
 477         addl    $12,%esp
 478 #endif  /* NNPX > 0 */
 479
 480 1:
 481         ret
 482
 483 /*
 484  * cpu_idle_restore()   (current thread in %eax on entry) (one-time execution)
 485  *
 486  *      Don't bother setting up any regs other then %ebp so backtraces
 487  *      don't die.  This restore function is used to bootstrap into the
 488  *      cpu_idle() LWKT only, after that cpu_lwkt_*() will be used for
 489  *      switching.
 490  *
 491  *      Clear TDF_RUNNING in old thread only after we've cleaned up %cr3.
 492  *
 493  *      If we are an AP we have to call ap_init() before jumping to
 494  *      cpu_idle().  ap_init() will synchronize with the BP and finish
 495  *      setting up various ncpu-dependant globaldata fields.  This may
 496  *      happen on UP as well as SMP if we happen to be simulating multiple
 497  *      cpus.
 498  */
 499 ENTRY(cpu_idle_restore)
 500         /* cli */
 501         movl    IdlePTD,%ecx
 502         movl    $0,%ebp
 503         pushl   $0
 504         movl    %ecx,%cr3
 505         andl    $~TDF_RUNNING,TD_FLAGS(%ebx)
 506         orl     $TDF_RUNNING,TD_FLAGS(%eax)
 507 #ifdef SMP
 508         cmpl    $0,PCPU(cpuid)
 509         je      1f
 510         call    ap_init
 511 1:
 512 #endif
 513         /*
 514          * ap_init can decide to enable interrupts early, but otherwise, or if
 515          * we are UP, do it here.
 516          */
 517         sti
 518         jmp     cpu_idle
 519
 520 /*
 521  * cpu_kthread_restore() (current thread is %eax on entry) (one-time execution)
 522  *
 523  *      Don't bother setting up any regs other then %ebp so backtraces
 524  *      don't die.  This restore function is used to bootstrap into an
 525  *      LWKT based kernel thread only.  cpu_lwkt_switch() will be used
 526  *      after this.
 527  *
 528  *      Since all of our context is on the stack we are reentrant and
 529  *      we can release our critical section and enable interrupts early.
 530  */
 531 ENTRY(cpu_kthread_restore)
 532         sti
 533         movl    IdlePTD,%ecx
 534         movl    TD_PCB(%eax),%edx
 535         movl    $0,%ebp
 536         movl    %ecx,%cr3
 537         andl    $~TDF_RUNNING,TD_FLAGS(%ebx)
 538         orl     $TDF_RUNNING,TD_FLAGS(%eax)
 539         subl    $TDPRI_CRIT,TD_PRI(%eax)
 540         popl    %eax            /* kthread exit function */
 541         pushl   PCB_EBX(%edx)   /* argument to ESI function */
 542         pushl   %eax            /* set exit func as return address */
 543         movl    PCB_ESI(%edx),%eax
 544         jmp     *%eax
 545
 546 /*
 547  * cpu_lwkt_switch()
 548  *
 549  *      Standard LWKT switching function.  Only non-scratch registers are
 550  *      saved and we don't bother with the MMU state or anything else.
 551  *
 552  *      This function is always called while in a critical section.
 553  *
 554  *      There is a one-instruction window where curthread is the new
 555  *      thread but %esp still points to the old thread's stack, but
 556  *      we are protected by a critical section so it is ok.
 557  *
 558  *      YYY BGL, SPL
 559  */
 560 ENTRY(cpu_lwkt_switch)
 561         pushl   %ebp    /* note: GDB hacked to locate ebp relative to td_sp */
 562         pushl   %ebx
 563         movl    PCPU(curthread),%ebx
 564         pushl   %esi
 565         pushl   %edi
 566         pushfl
 567         /* warning: adjust movl into %eax below if you change the pushes */
 568
 569 #if NNPX > 0
 570         /*
 571          * Save the FP state if we have used the FP.  Note that calling
 572          * npxsave will NULL out PCPU(npxthread).
 573          *
 574          * We have to deal with the FP state for LWKT threads in case they
 575          * happen to get preempted or block while doing an optimized
 576          * bzero/bcopy/memcpy.
 577          */
 578         cmpl    %ebx,PCPU(npxthread)
 579         jne     1f
 580         pushl   TD_SAVEFPU(%ebx)
 581         call    npxsave                 /* do it in a big C function */
 582         addl    $4,%esp                 /* EAX, ECX, EDX trashed */
 583 1:
 584 #endif  /* NNPX > 0 */
 585
 586         movl    4+20(%esp),%eax         /* switch to this thread */
 587         pushl   $cpu_lwkt_restore
 588         movl    %esp,TD_SP(%ebx)
 589         movl    %eax,PCPU(curthread)
 590         movl    TD_SP(%eax),%esp
 591
 592         /*
 593          * eax contains new thread, ebx contains old thread.
 594          */
 595         ret
 596
 597 /*
 598  * cpu_lwkt_restore()   (current thread in %eax on entry)
 599  *
 600  *      Standard LWKT restore function.  This function is always called
 601  *      while in a critical section.
 602  *
 603  *      Warning: due to preemption the restore function can be used to
 604  *      'return' to the original thread.  Interrupt disablement must be
 605  *      protected through the switch so we cannot run splz here.
 606  *
 607  *      YYY we theoretically do not need to load IdlePTD into cr3, but if
 608  *      so we need a way to detect when the PTD we are using is being
 609  *      deleted due to a process exiting.
 610  */
 611 ENTRY(cpu_lwkt_restore)
 612         movl    IdlePTD,%ecx    /* YYY borrow but beware desched/cpuchg/exit */
 613         movl    %cr3,%edx
 614         cmpl    %ecx,%edx
 615         je      1f
 616         movl    %ecx,%cr3
 617 1:
 618         andl    $~TDF_RUNNING,TD_FLAGS(%ebx)
 619         orl     $TDF_RUNNING,TD_FLAGS(%eax)
 620         popfl
 621         popl    %edi
 622         popl    %esi
 623         popl    %ebx
 624         popl    %ebp
 625         ret
 626