| 1 | |
| 2 | * Context tracking: Probe on high level context boundaries such as kernel
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| 3 | * and userspace. This includes syscalls and exceptions entry/exit.
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| 4 | *
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| 5 | * This is used by RCU to remove its dependency on the timer tick while a CPU
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| 6 | * runs in userspace.
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| 7 | *
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| 8 | * Started by Frederic Weisbecker:
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| 9 | *
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| 10 | * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <[email protected]>
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| 11 | *
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| 12 | * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
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| 13 | * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
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| 14 | *
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| 15 |
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| 16 |
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| 17 | #include <linux/context_tracking.h>
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| 18 | #include <linux/kvm_host.h>
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| 19 | #include <linux/rcupdate.h>
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| 20 | #include <linux/sched.h>
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| 21 | #include <linux/hardirq.h>
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| 22 | #include <linux/export.h>
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| 23 |
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| 24 | DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
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| 25 | #ifdef CONFIG_CONTEXT_TRACKING_FORCE
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| 26 | .active = true,
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| 27 | #endif
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| 28 | };
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| 29 |
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| 30 | |
| 31 | * user_enter - Inform the context tracking that the CPU is going to
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| 32 | * enter userspace mode.
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| 33 | *
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| 34 | * This function must be called right before we switch from the kernel
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| 35 | * to userspace, when it's guaranteed the remaining kernel instructions
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| 36 | * to execute won't use any RCU read side critical section because this
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| 37 | * function sets RCU in extended quiescent state.
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| 38 |
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| 39 | void user_enter(void)
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| 40 | {
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| 41 | unsigned long flags;
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| 42 |
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| 43 | |
| 44 | * Some contexts may involve an exception occuring in an irq,
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| 45 | * leading to that nesting:
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| 46 | * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
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| 47 | * This would mess up the dyntick_nesting count though. And rcu_irq_*()
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| 48 | * helpers are enough to protect RCU uses inside the exception. So
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| 49 | * just return immediately if we detect we are in an IRQ.
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| 50 |
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| 51 | if (in_interrupt())
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| 52 | return;
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| 53 |
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| 54 |
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| 55 | WARN_ON_ONCE(!current->mm);
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| 56 |
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| 57 | local_irq_save(flags);
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| 58 | if (__this_cpu_read(context_tracking.active) &&
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| 59 | __this_cpu_read(context_tracking.state) != IN_USER) {
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| 60 | |
| 61 | * At this stage, only low level arch entry code remains and
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| 62 | * then we'll run in userspace. We can assume there won't be
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| 63 | * any RCU read-side critical section until the next call to
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| 64 | * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
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| 65 | * on the tick.
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| 66 |
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| 67 | vtime_user_enter(current);
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| 68 | rcu_user_enter();
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| 69 | __this_cpu_write(context_tracking.state, IN_USER);
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| 70 | }
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| 71 | local_irq_restore(flags);
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| 72 | }
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| 73 |
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| 74 |
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| 75 | |
| 76 | * user_exit - Inform the context tracking that the CPU is
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| 77 | * exiting userspace mode and entering the kernel.
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| 78 | *
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| 79 | * This function must be called after we entered the kernel from userspace
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| 80 | * before any use of RCU read side critical section. This potentially include
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| 81 | * any high level kernel code like syscalls, exceptions, signal handling, etc...
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| 82 | *
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| 83 | * This call supports re-entrancy. This way it can be called from any exception
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| 84 | * handler without needing to know if we came from userspace or not.
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| 85 |
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| 86 | void user_exit(void)
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| 87 | {
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| 88 | unsigned long flags;
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| 89 |
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| 90 | if (in_interrupt())
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| 91 | return;
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| 92 |
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| 93 | local_irq_save(flags);
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| 94 | if (__this_cpu_read(context_tracking.state) == IN_USER) {
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| 95 | |
| 96 | * We are going to run code that may use RCU. Inform
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| 97 | * RCU core about that (ie: we may need the tick again).
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| 98 |
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| 99 | rcu_user_exit();
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| 100 | vtime_user_exit(current);
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| 101 | __this_cpu_write(context_tracking.state, IN_KERNEL);
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| 102 | }
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| 103 | local_irq_restore(flags);
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| 104 | }
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| 105 |
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| 106 | void guest_enter(void)
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| 107 | {
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| 108 | if (vtime_accounting_enabled())
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| 109 | vtime_guest_enter(current);
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| 110 | else
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| 111 | __guest_enter();
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| 112 | }
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| 113 | EXPORT_SYMBOL_GPL(guest_enter);
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| 114 |
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| 115 | void guest_exit(void)
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| 116 | {
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| 117 | if (vtime_accounting_enabled())
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| 118 | vtime_guest_exit(current);
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| 119 | else
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| 120 | __guest_exit();
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| 121 | }
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| 122 | EXPORT_SYMBOL_GPL(guest_exit);
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| 123 |
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| 124 |
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| 125 | |
| 126 | * context_tracking_task_switch - context switch the syscall callbacks
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| 127 | * @prev: the task that is being switched out
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| 128 | * @next: the task that is being switched in
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| 129 | *
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| 130 | * The context tracking uses the syscall slow path to implement its user-kernel
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| 131 | * boundaries probes on syscalls. This way it doesn't impact the syscall fast
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| 132 | * path on CPUs that don't do context tracking.
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| 133 | *
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| 134 | * But we need to clear the flag on the previous task because it may later
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| 135 | * migrate to some CPU that doesn't do the context tracking. As such the TIF
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| 136 | * flag may not be desired there.
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| 137 |
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| 138 | void context_tracking_task_switch(struct task_struct *prev,
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| 139 | struct task_struct *next)
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| 140 | {
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| 141 | if (__this_cpu_read(context_tracking.active)) {
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| 142 | clear_tsk_thread_flag(prev, TIF_NOHZ);
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| 143 | set_tsk_thread_flag(next, TIF_NOHZ);
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| 144 | }
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| 145 | }
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