x86_64/gen/cpu_number.s

   1 /*
   2  * Copyright (c) 2008 Apple Inc. All rights reserved.
   3  *
   4  * @APPLE_OSREFERENCE_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. The rights granted to you under the License
  10  * may not be used to create, or enable the creation or redistribution of,
  11  * unlawful or unlicensed copies of an Apple operating system, or to
  12  * circumvent, violate, or enable the circumvention or violation of, any
  13  * terms of an Apple operating system software license agreement.
  14  *
  15  * Please obtain a copy of the License at
  16  * http://www.opensource.apple.com/apsl/ and read it before using this file.
  17  *
  18  * The Original Code and all software distributed under the License are
  19  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  20  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  21  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  22  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
  23  * Please see the License for the specific language governing rights and
  24  * limitations under the License.
  25  *
  26  * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
  27  */
  28
  29 /*
  30  * This routine provides fast access to the logical cpu number
  31  * of the calling processor assuming no pre-emption occurs. This number
  32  * is encoded in the bottom 12-bits of the limit field of the IDTR (the
  33  * Interrupt Descriptor Table Register). The SIDT instruction is used in
  34  * userspace to read this register and thus to gain access to the cpu number.
  35  * The IDTR is loaded by the kernel for each processor at startup - see
  36  * osfmk/i386/mp_desc.c.
  37  */
  38
  39 /* return logical cpu number in %rax */
  40
  41         .private_extern _cpu_number
  42 _cpu_number:
  43         push    %rbp
  44         mov     %rsp,%rbp
  45         sub     $16,%rsp                // space to read IDTR
  46
  47         sidt    (%rsp)                  // store limit:base on stack
  48         movw    (%rsp), %ax             // get limit
  49         and     $0xfff, %rax            // mask off lower 12 bits to return
  50
  51         mov     %rbp,%rsp
  52         pop     %rbp
  53         ret