i386/pthreads/preempt.s

   1 /*
   2  * Copyright (c) 2010 Apple Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * This file contains Original Code and/or Modifications of Original Code
   7  * as defined in and that are subject to the Apple Public Source License
   8  * Version 2.0 (the 'License'). You may not use this file except in
   9  * compliance with the License. Please obtain a copy of the License at
  10  * http://www.opensource.apple.com/apsl/ and read it before using this
  11  * file.
  12  *
  13  * The Original Code and all software distributed under the License are
  14  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  15  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  16  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
  18  * Please see the License for the specific language governing rights and
  19  * limitations under the License.
  20  *
  21  * @APPLE_LICENSE_HEADER_END@
  22  */
  23
  24 #include <machine/cpu_capabilities.h>
  25 #include <mach/i386/syscall_sw.h>
  26
  27 /* Subroutine to make a preempt syscall.  Called when we notice %ebx is
  28  * nonzero after returning from a PFZ subroutine.
  29  * When we enter kernel:
  30  *      %edx = return address
  31  *      %ecx = stack ptr
  32  * Destroys %eax, %ecx, and %edx.
  33  */
  34         .align  4
  35         .private_extern _preempt
  36 _preempt:
  37         popl    %edx            // get return address
  38         movl    %esp,%ecx       // save stack ptr here
  39         movl    $(-58),%eax     /* 58 = pfz_exit */
  40         xorl    %ebx,%ebx       // clear "preemption pending" flag
  41         sysenter
  42
  43
  44 /* Subroutine to back off if we cannot get the spinlock.  Called
  45  * after a few attempts inline in the PFZ subroutines.  This code is
  46  * not in the PFZ.
  47  *      %edi = ptr to queue head structure
  48  *      %ebx = preemption flag (nonzero if preemption pending)
  49  * Destroys %eax.
  50  */
  51
  52         .align  4
  53         .private_extern _backoff
  54 _backoff:
  55         testl   %ebx,%ebx       // does kernel want to preempt us?
  56         jz      1f              // no
  57         xorl    %ebx,%ebx       // yes, clear flag
  58         pushl   %edx            // preserve regs used by preempt syscall
  59         pushl   %ecx
  60         call    _preempt
  61         popl    %ecx
  62         popl    %edx
  63 1:
  64         pause                   // SMT-friendly backoff
  65         cmpl    $0,8(%edi)      // sniff the lockword
  66         jnz     1b              // loop if still taken
  67         ret                     // lockword is free, so reenter PFZ