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1 /*
2 * Copyright (c) 2009
3 * The President and Fellows of Harvard College.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 * notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 * notice, this list of conditions and the following disclaimer in the
12 * documentation and/or other materials provided with the distribution.
13 * 3. Neither the name of the University nor the names of its contributors
14 * may be used to endorse or promote products derived from this software
15 * without specific prior written permission.
16 *
17 * THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY AND CONTRIBUTORS ``AS IS'' AND
18 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
19 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
20 * ARE DISCLAIMED. IN NO EVENT SHALL THE UNIVERSITY OR CONTRIBUTORS BE LIABLE
21 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
22 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
23 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
24 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
25 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
26 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
27 * SUCH DAMAGE.
28 */
29
30 #ifndef _MIPS_CURRENT_H_
31 #define _MIPS_CURRENT_H_
32
33
34 /*
35 * Macro for current thread, or alternatively current cpu.
36 *
37 * This file should only be included via <current.h> (q.v.)
38 *
39 * These are machine-dependent because on some platforms it is
40 * better/easier to keep track of curcpu and make curthread be
41 * curcpu->c_curthread, and on others to keep track of curthread and
42 * make curcpu be curthread->t_cpu.
43 *
44 * Either way we don't want retrieving curthread or curcpu to be
45 * expensive; digging around in system board registers and whatnot is
46 * not a very good idea. So we want to keep either curthread or curcpu
47 * on-chip somewhere in some fashion.
48 *
49 * There are various possible approaches; for example, one might use
50 * the MMU on each CPU to map that CPU's cpu structure to a fixed
51 * virtual address that's the same on all CPUs. Then curcpu can be a
52 * constant. (But one has to remember to use curcpu->c_self as the
53 * canonical form of the pointer anywhere that's visible to other
54 * CPUs.) Another approach is to reserve a register to hold curthread.
55 *
56 * On mips there's an architectural issue that informs this choice:
57 * there's no easy way to find the current cpu, the current thread, or
58 * even the kernel stack of the current thread when entering the
59 * kernel at trap time. (On most CPUs there's a canonical way to find
60 * at least the stack.)
61 *
62 * Therefore we do the following:
63 *
64 * - We misuse a kernel-settable field of a nonessential MMU register
65 * to hold the CPU number.
66 *
67 * - On trap entry we use this number to index an array that gets us
68 * both the kernel stack and curthread.
69 *
70 * - We tell the compiler not to use the s7 register and keep
71 * curthread there.
72 *
73 * Note that if you want to change this scheme to use a different
74 * register, or change to a different scheme, you need to touch three
75 * places: here, the mips-specific kernel CFLAGS in the makefiles, and
76 * the trap entry and return code.
77 */
78
79 register struct thread *curthread asm("$23"); /* s7 register */
80 #undef __NEED_CURTHREAD
81 #define __NEED_CURCPU
82
83 /* For how we've defined it, curthread gets set first, then curcpu. */
84 #define INIT_CURCPU(cpu, thread) (curthread = (thread), curcpu = (cpu))
85
86 #endif /* _MIPS_CURRENT_H_ */