util/coroutine-ucontext.c

   1 /*
   2  * ucontext coroutine initialization code
   3  *
   4  * Copyright (C) 2006  Anthony Liguori <anthony@codemonkey.ws>
   5  * Copyright (C) 2011  Kevin Wolf <kwolf@redhat.com>
   6  *
   7  * This library is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU Lesser General Public
   9  * License as published by the Free Software Foundation; either
  10  * version 2.0 of the License, or (at your option) any later version.
  11  *
  12  * This library is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15  * Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public
  18  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  19  */
  20
  21 /* XXX Is there a nicer way to disable glibc's stack check for longjmp? */
  22 #ifdef _FORTIFY_SOURCE
  23 #undef _FORTIFY_SOURCE
  24 #endif
  25 #include "qemu/osdep.h"
  26 #include <ucontext.h>
  27 #include "qemu-common.h"
  28 #include "qemu/coroutine_int.h"
  29
  30 #ifdef CONFIG_VALGRIND_H
  31 #include <valgrind/valgrind.h>
  32 #endif
  33
  34 typedef struct {
  35     Coroutine base;
  36     void *stack;
  37     sigjmp_buf env;
  38
  39 #ifdef CONFIG_VALGRIND_H
  40     unsigned int valgrind_stack_id;
  41 #endif
  42
  43 } CoroutineUContext;
  44
  45 /**
  46  * Per-thread coroutine bookkeeping
  47  */
  48 static __thread CoroutineUContext leader;
  49 static __thread Coroutine *current;
  50
  51 /*
  52  * va_args to makecontext() must be type 'int', so passing
  53  * the pointer we need may require several int args. This
  54  * union is a quick hack to let us do that
  55  */
  56 union cc_arg {
  57     void *p;
  58     int i[2];
  59 };
  60
  61 static void coroutine_trampoline(int i0, int i1)
  62 {
  63     union cc_arg arg;
  64     CoroutineUContext *self;
  65     Coroutine *co;
  66
  67     arg.i[0] = i0;
  68     arg.i[1] = i1;
  69     self = arg.p;
  70     co = &self->base;
  71
  72     /* Initialize longjmp environment and switch back the caller */
  73     if (!sigsetjmp(self->env, 0)) {
  74         siglongjmp(*(sigjmp_buf *)co->entry_arg, 1);
  75     }
  76
  77     while (true) {
  78         co->entry(co->entry_arg);
  79         qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
  80     }
  81 }
  82
  83 Coroutine *qemu_coroutine_new(void)
  84 {
  85     const size_t stack_size = 1 << 20;
  86     CoroutineUContext *co;
  87     ucontext_t old_uc, uc;
  88     sigjmp_buf old_env;
  89     union cc_arg arg = {0};
  90
  91     /* The ucontext functions preserve signal masks which incurs a
  92      * system call overhead.  sigsetjmp(buf, 0)/siglongjmp() does not
  93      * preserve signal masks but only works on the current stack.
  94      * Since we need a way to create and switch to a new stack, use
  95      * the ucontext functions for that but sigsetjmp()/siglongjmp() for
  96      * everything else.
  97      */
  98
  99     if (getcontext(&uc) == -1) {
 100         abort();
 101     }
 102
 103     co = g_malloc0(sizeof(*co));
 104     co->stack = g_malloc(stack_size);
 105     co->base.entry_arg = &old_env; /* stash away our jmp_buf */
 106
 107     uc.uc_link = &old_uc;
 108     uc.uc_stack.ss_sp = co->stack;
 109     uc.uc_stack.ss_size = stack_size;
 110     uc.uc_stack.ss_flags = 0;
 111
 112 #ifdef CONFIG_VALGRIND_H
 113     co->valgrind_stack_id =
 114         VALGRIND_STACK_REGISTER(co->stack, co->stack + stack_size);
 115 #endif
 116
 117     arg.p = co;
 118
 119     makecontext(&uc, (void (*)(void))coroutine_trampoline,
 120                 2, arg.i[0], arg.i[1]);
 121
 122     /* swapcontext() in, siglongjmp() back out */
 123     if (!sigsetjmp(old_env, 0)) {
 124         swapcontext(&old_uc, &uc);
 125     }
 126     return &co->base;
 127 }
 128
 129 #ifdef CONFIG_VALGRIND_H
 130 #ifdef CONFIG_PRAGMA_DIAGNOSTIC_AVAILABLE
 131 /* Work around an unused variable in the valgrind.h macro... */
 132 #pragma GCC diagnostic push
 133 #pragma GCC diagnostic ignored "-Wunused-but-set-variable"
 134 #endif
 135 static inline void valgrind_stack_deregister(CoroutineUContext *co)
 136 {
 137     VALGRIND_STACK_DEREGISTER(co->valgrind_stack_id);
 138 }
 139 #ifdef CONFIG_PRAGMA_DIAGNOSTIC_AVAILABLE
 140 #pragma GCC diagnostic pop
 141 #endif
 142 #endif
 143
 144 void qemu_coroutine_delete(Coroutine *co_)
 145 {
 146     CoroutineUContext *co = DO_UPCAST(CoroutineUContext, base, co_);
 147
 148 #ifdef CONFIG_VALGRIND_H
 149     valgrind_stack_deregister(co);
 150 #endif
 151
 152     g_free(co->stack);
 153     g_free(co);
 154 }
 155
 156 /* This function is marked noinline to prevent GCC from inlining it
 157  * into coroutine_trampoline(). If we allow it to do that then it
 158  * hoists the code to get the address of the TLS variable "current"
 159  * out of the while() loop. This is an invalid transformation because
 160  * the sigsetjmp() call may be called when running thread A but
 161  * return in thread B, and so we might be in a different thread
 162  * context each time round the loop.
 163  */
 164 CoroutineAction __attribute__((noinline))
 165 qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
 166                       CoroutineAction action)
 167 {
 168     CoroutineUContext *from = DO_UPCAST(CoroutineUContext, base, from_);
 169     CoroutineUContext *to = DO_UPCAST(CoroutineUContext, base, to_);
 170     int ret;
 171
 172     current = to_;
 173
 174     ret = sigsetjmp(from->env, 0);
 175     if (ret == 0) {
 176         siglongjmp(to->env, action);
 177     }
 178     return ret;
 179 }
 180
 181 Coroutine *qemu_coroutine_self(void)
 182 {
 183     if (!current) {
 184         current = &leader.base;
 185     }
 186     return current;
 187 }
 188
 189 bool qemu_in_coroutine(void)
 190 {
 191     return current && current->caller;
 192 }