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279 lines
9.5 KiB
279 lines
9.5 KiB
Entry/exit handling for exceptions, interrupts, syscalls and KVM |
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================================================================ |
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All transitions between execution domains require state updates which are |
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subject to strict ordering constraints. State updates are required for the |
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following: |
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* Lockdep |
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* RCU / Context tracking |
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* Preemption counter |
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* Tracing |
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* Time accounting |
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The update order depends on the transition type and is explained below in |
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the transition type sections: `Syscalls`_, `KVM`_, `Interrupts and regular |
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exceptions`_, `NMI and NMI-like exceptions`_. |
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Non-instrumentable code - noinstr |
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--------------------------------- |
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Most instrumentation facilities depend on RCU, so intrumentation is prohibited |
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for entry code before RCU starts watching and exit code after RCU stops |
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watching. In addition, many architectures must save and restore register state, |
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which means that (for example) a breakpoint in the breakpoint entry code would |
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overwrite the debug registers of the initial breakpoint. |
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Such code must be marked with the 'noinstr' attribute, placing that code into a |
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special section inaccessible to instrumentation and debug facilities. Some |
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functions are partially instrumentable, which is handled by marking them |
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noinstr and using instrumentation_begin() and instrumentation_end() to flag the |
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instrumentable ranges of code: |
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.. code-block:: c |
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noinstr void entry(void) |
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{ |
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handle_entry(); // <-- must be 'noinstr' or '__always_inline' |
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... |
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instrumentation_begin(); |
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handle_context(); // <-- instrumentable code |
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instrumentation_end(); |
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... |
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handle_exit(); // <-- must be 'noinstr' or '__always_inline' |
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} |
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This allows verification of the 'noinstr' restrictions via objtool on |
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supported architectures. |
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Invoking non-instrumentable functions from instrumentable context has no |
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restrictions and is useful to protect e.g. state switching which would |
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cause malfunction if instrumented. |
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All non-instrumentable entry/exit code sections before and after the RCU |
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state transitions must run with interrupts disabled. |
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Syscalls |
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-------- |
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Syscall-entry code starts in assembly code and calls out into low-level C code |
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after establishing low-level architecture-specific state and stack frames. This |
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low-level C code must not be instrumented. A typical syscall handling function |
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invoked from low-level assembly code looks like this: |
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.. code-block:: c |
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noinstr void syscall(struct pt_regs *regs, int nr) |
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{ |
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arch_syscall_enter(regs); |
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nr = syscall_enter_from_user_mode(regs, nr); |
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instrumentation_begin(); |
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if (!invoke_syscall(regs, nr) && nr != -1) |
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result_reg(regs) = __sys_ni_syscall(regs); |
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instrumentation_end(); |
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syscall_exit_to_user_mode(regs); |
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} |
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syscall_enter_from_user_mode() first invokes enter_from_user_mode() which |
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establishes state in the following order: |
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* Lockdep |
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* RCU / Context tracking |
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* Tracing |
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and then invokes the various entry work functions like ptrace, seccomp, audit, |
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syscall tracing, etc. After all that is done, the instrumentable invoke_syscall |
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function can be invoked. The instrumentable code section then ends, after which |
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syscall_exit_to_user_mode() is invoked. |
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syscall_exit_to_user_mode() handles all work which needs to be done before |
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returning to user space like tracing, audit, signals, task work etc. After |
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that it invokes exit_to_user_mode() which again handles the state |
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transition in the reverse order: |
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* Tracing |
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* RCU / Context tracking |
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* Lockdep |
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syscall_enter_from_user_mode() and syscall_exit_to_user_mode() are also |
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available as fine grained subfunctions in cases where the architecture code |
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has to do extra work between the various steps. In such cases it has to |
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ensure that enter_from_user_mode() is called first on entry and |
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exit_to_user_mode() is called last on exit. |
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Do not nest syscalls. Nested systcalls will cause RCU and/or context tracking |
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to print a warning. |
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KVM |
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--- |
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Entering or exiting guest mode is very similar to syscalls. From the host |
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kernel point of view the CPU goes off into user space when entering the |
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guest and returns to the kernel on exit. |
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kvm_guest_enter_irqoff() is a KVM-specific variant of exit_to_user_mode() |
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and kvm_guest_exit_irqoff() is the KVM variant of enter_from_user_mode(). |
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The state operations have the same ordering. |
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Task work handling is done separately for guest at the boundary of the |
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vcpu_run() loop via xfer_to_guest_mode_handle_work() which is a subset of |
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the work handled on return to user space. |
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Do not nest KVM entry/exit transitions because doing so is nonsensical. |
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Interrupts and regular exceptions |
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--------------------------------- |
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Interrupts entry and exit handling is slightly more complex than syscalls |
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and KVM transitions. |
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If an interrupt is raised while the CPU executes in user space, the entry |
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and exit handling is exactly the same as for syscalls. |
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If the interrupt is raised while the CPU executes in kernel space the entry and |
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exit handling is slightly different. RCU state is only updated when the |
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interrupt is raised in the context of the CPU's idle task. Otherwise, RCU will |
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already be watching. Lockdep and tracing have to be updated unconditionally. |
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irqentry_enter() and irqentry_exit() provide the implementation for this. |
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The architecture-specific part looks similar to syscall handling: |
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.. code-block:: c |
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noinstr void interrupt(struct pt_regs *regs, int nr) |
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{ |
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arch_interrupt_enter(regs); |
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state = irqentry_enter(regs); |
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instrumentation_begin(); |
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irq_enter_rcu(); |
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invoke_irq_handler(regs, nr); |
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irq_exit_rcu(); |
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instrumentation_end(); |
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irqentry_exit(regs, state); |
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} |
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Note that the invocation of the actual interrupt handler is within a |
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irq_enter_rcu() and irq_exit_rcu() pair. |
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irq_enter_rcu() updates the preemption count which makes in_hardirq() |
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return true, handles NOHZ tick state and interrupt time accounting. This |
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means that up to the point where irq_enter_rcu() is invoked in_hardirq() |
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returns false. |
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irq_exit_rcu() handles interrupt time accounting, undoes the preemption |
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count update and eventually handles soft interrupts and NOHZ tick state. |
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In theory, the preemption count could be updated in irqentry_enter(). In |
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practice, deferring this update to irq_enter_rcu() allows the preemption-count |
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code to be traced, while also maintaining symmetry with irq_exit_rcu() and |
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irqentry_exit(), which are described in the next paragraph. The only downside |
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is that the early entry code up to irq_enter_rcu() must be aware that the |
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preemption count has not yet been updated with the HARDIRQ_OFFSET state. |
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Note that irq_exit_rcu() must remove HARDIRQ_OFFSET from the preemption count |
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before it handles soft interrupts, whose handlers must run in BH context rather |
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than irq-disabled context. In addition, irqentry_exit() might schedule, which |
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also requires that HARDIRQ_OFFSET has been removed from the preemption count. |
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Even though interrupt handlers are expected to run with local interrupts |
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disabled, interrupt nesting is common from an entry/exit perspective. For |
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example, softirq handling happens within an irqentry_{enter,exit}() block with |
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local interrupts enabled. Also, although uncommon, nothing prevents an |
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interrupt handler from re-enabling interrupts. |
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Interrupt entry/exit code doesn't strictly need to handle reentrancy, since it |
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runs with local interrupts disabled. But NMIs can happen anytime, and a lot of |
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the entry code is shared between the two. |
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NMI and NMI-like exceptions |
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--------------------------- |
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NMIs and NMI-like exceptions (machine checks, double faults, debug |
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interrupts, etc.) can hit any context and must be extra careful with |
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the state. |
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State changes for debug exceptions and machine-check exceptions depend on |
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whether these exceptions happened in user-space (breakpoints or watchpoints) or |
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in kernel mode (code patching). From user-space, they are treated like |
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interrupts, while from kernel mode they are treated like NMIs. |
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NMIs and other NMI-like exceptions handle state transitions without |
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distinguishing between user-mode and kernel-mode origin. |
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The state update on entry is handled in irqentry_nmi_enter() which updates |
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state in the following order: |
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* Preemption counter |
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* Lockdep |
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* RCU / Context tracking |
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* Tracing |
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The exit counterpart irqentry_nmi_exit() does the reverse operation in the |
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reverse order. |
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Note that the update of the preemption counter has to be the first |
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operation on enter and the last operation on exit. The reason is that both |
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lockdep and RCU rely on in_nmi() returning true in this case. The |
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preemption count modification in the NMI entry/exit case must not be |
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traced. |
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Architecture-specific code looks like this: |
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.. code-block:: c |
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noinstr void nmi(struct pt_regs *regs) |
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{ |
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arch_nmi_enter(regs); |
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state = irqentry_nmi_enter(regs); |
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instrumentation_begin(); |
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nmi_handler(regs); |
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instrumentation_end(); |
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irqentry_nmi_exit(regs); |
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} |
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and for e.g. a debug exception it can look like this: |
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.. code-block:: c |
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noinstr void debug(struct pt_regs *regs) |
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{ |
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arch_nmi_enter(regs); |
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debug_regs = save_debug_regs(); |
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if (user_mode(regs)) { |
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state = irqentry_enter(regs); |
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instrumentation_begin(); |
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user_mode_debug_handler(regs, debug_regs); |
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instrumentation_end(); |
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irqentry_exit(regs, state); |
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} else { |
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state = irqentry_nmi_enter(regs); |
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instrumentation_begin(); |
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kernel_mode_debug_handler(regs, debug_regs); |
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instrumentation_end(); |
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irqentry_nmi_exit(regs, state); |
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} |
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} |
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There is no combined irqentry_nmi_if_kernel() function available as the |
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above cannot be handled in an exception-agnostic way. |
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NMIs can happen in any context. For example, an NMI-like exception triggered |
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while handling an NMI. So NMI entry code has to be reentrant and state updates |
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need to handle nesting.
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