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The typical steps with context tracking are:
1) Task runs in userspace
2) Task enters the kernel (syscall/exception/IRQ)
3) Task switches from context tracking state CONTEXT_USER to
CONTEXT_KERNEL (user_exit())
4) Task does stuff in kernel
5) Task switches from context tracking state CONTEXT_KERNEL to
CONTEXT_USER (user_enter())
6) Task exits the kernel
If an exception fires between 5) and 6), the pt_regs and the context
tracking disagree on the context of the faulted/trapped instruction.
CONTEXT_KERNEL must be set before the exception handler, that's
unconditional for those handlers that want to be able to call into
schedule(), but CONTEXT_USER must be restored when the exception exits
whereas pt_regs tells that we are resuming to kernel space.
This can't be fixed with storing the context tracking state in a per-cpu
or per-task variable since another exception may fire onto the current
one and overwrite the saved state. Also the task can schedule. So it
has to be stored in a per task stack.
This is how exception_enter()/exception_exit() paper over the problem:
5) Task switches from context tracking state CONTEXT_KERNEL to
CONTEXT_USER (user_enter())
5.1) Exception fires
5.2) prev_state = exception_enter() // save CONTEXT_USER to prev_state
// and set CONTEXT_KERNEL
5.3) Exception handler
5.4) exception_enter(prev_state) // restore CONTEXT_USER
5.5) Exception resumes
6) Task exits the kernel
The condition to live without exception_enter()/exception_exit() is to
forbid exceptions and IRQs between 2) and 3) and between 5) and 6), or if
any is allowed to trigger, it won't call into context tracking, eg: NMIs,
and it won't schedule. These requirements are met by architectures
supporting CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK and those can
therefore afford not to implement this hack.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201117151637.259084-3-frederic@kernel.org
181 lines
4.7 KiB
C
181 lines
4.7 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_CONTEXT_TRACKING_H
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#define _LINUX_CONTEXT_TRACKING_H
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#include <linux/sched.h>
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#include <linux/vtime.h>
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#include <linux/context_tracking_state.h>
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#include <linux/instrumentation.h>
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#include <asm/ptrace.h>
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#ifdef CONFIG_CONTEXT_TRACKING
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extern void context_tracking_cpu_set(int cpu);
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/* Called with interrupts disabled. */
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extern void __context_tracking_enter(enum ctx_state state);
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extern void __context_tracking_exit(enum ctx_state state);
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extern void context_tracking_enter(enum ctx_state state);
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extern void context_tracking_exit(enum ctx_state state);
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extern void context_tracking_user_enter(void);
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extern void context_tracking_user_exit(void);
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static inline void user_enter(void)
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{
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if (context_tracking_enabled())
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context_tracking_enter(CONTEXT_USER);
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}
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static inline void user_exit(void)
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{
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if (context_tracking_enabled())
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context_tracking_exit(CONTEXT_USER);
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}
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/* Called with interrupts disabled. */
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static __always_inline void user_enter_irqoff(void)
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{
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if (context_tracking_enabled())
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__context_tracking_enter(CONTEXT_USER);
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}
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static __always_inline void user_exit_irqoff(void)
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{
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if (context_tracking_enabled())
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__context_tracking_exit(CONTEXT_USER);
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}
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static inline enum ctx_state exception_enter(void)
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{
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enum ctx_state prev_ctx;
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if (IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) ||
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!context_tracking_enabled())
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return 0;
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prev_ctx = this_cpu_read(context_tracking.state);
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if (prev_ctx != CONTEXT_KERNEL)
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context_tracking_exit(prev_ctx);
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return prev_ctx;
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}
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static inline void exception_exit(enum ctx_state prev_ctx)
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{
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if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) &&
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context_tracking_enabled()) {
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if (prev_ctx != CONTEXT_KERNEL)
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context_tracking_enter(prev_ctx);
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}
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}
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/**
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* ct_state() - return the current context tracking state if known
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*
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* Returns the current cpu's context tracking state if context tracking
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* is enabled. If context tracking is disabled, returns
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* CONTEXT_DISABLED. This should be used primarily for debugging.
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*/
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static __always_inline enum ctx_state ct_state(void)
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{
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return context_tracking_enabled() ?
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this_cpu_read(context_tracking.state) : CONTEXT_DISABLED;
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}
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#else
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static inline void user_enter(void) { }
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static inline void user_exit(void) { }
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static inline void user_enter_irqoff(void) { }
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static inline void user_exit_irqoff(void) { }
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static inline enum ctx_state exception_enter(void) { return 0; }
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static inline void exception_exit(enum ctx_state prev_ctx) { }
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static inline enum ctx_state ct_state(void) { return CONTEXT_DISABLED; }
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#endif /* !CONFIG_CONTEXT_TRACKING */
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#define CT_WARN_ON(cond) WARN_ON(context_tracking_enabled() && (cond))
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#ifdef CONFIG_CONTEXT_TRACKING_FORCE
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extern void context_tracking_init(void);
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#else
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static inline void context_tracking_init(void) { }
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#endif /* CONFIG_CONTEXT_TRACKING_FORCE */
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#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
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/* must be called with irqs disabled */
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static __always_inline void guest_enter_irqoff(void)
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{
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instrumentation_begin();
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if (vtime_accounting_enabled_this_cpu())
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vtime_guest_enter(current);
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else
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current->flags |= PF_VCPU;
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instrumentation_end();
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if (context_tracking_enabled())
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__context_tracking_enter(CONTEXT_GUEST);
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/* KVM does not hold any references to rcu protected data when it
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* switches CPU into a guest mode. In fact switching to a guest mode
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* is very similar to exiting to userspace from rcu point of view. In
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* addition CPU may stay in a guest mode for quite a long time (up to
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* one time slice). Lets treat guest mode as quiescent state, just like
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* we do with user-mode execution.
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*/
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if (!context_tracking_enabled_this_cpu()) {
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instrumentation_begin();
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rcu_virt_note_context_switch(smp_processor_id());
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instrumentation_end();
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}
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}
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static __always_inline void guest_exit_irqoff(void)
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{
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if (context_tracking_enabled())
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__context_tracking_exit(CONTEXT_GUEST);
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instrumentation_begin();
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if (vtime_accounting_enabled_this_cpu())
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vtime_guest_exit(current);
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else
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current->flags &= ~PF_VCPU;
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instrumentation_end();
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}
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#else
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static __always_inline void guest_enter_irqoff(void)
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{
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/*
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* This is running in ioctl context so its safe
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* to assume that it's the stime pending cputime
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* to flush.
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*/
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instrumentation_begin();
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vtime_account_kernel(current);
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current->flags |= PF_VCPU;
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rcu_virt_note_context_switch(smp_processor_id());
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instrumentation_end();
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}
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static __always_inline void guest_exit_irqoff(void)
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{
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instrumentation_begin();
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/* Flush the guest cputime we spent on the guest */
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vtime_account_kernel(current);
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current->flags &= ~PF_VCPU;
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instrumentation_end();
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}
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#endif /* CONFIG_VIRT_CPU_ACCOUNTING_GEN */
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static inline void guest_exit(void)
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{
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unsigned long flags;
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local_irq_save(flags);
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guest_exit_irqoff();
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local_irq_restore(flags);
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}
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#endif
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