From bf026e2e316ba57135b70e8ce591276239c7b2cf Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Mon, 10 Jan 2022 11:50:43 +0100 Subject: Documentation: Fill the gaps about entry/noinstr constraints The entry/exit handling for exceptions, interrupts, syscalls and KVM is not really documented except for some comments. Fill the gaps. Signed-off-by: Thomas Gleixner Signed-off-by: Nicolas Saenz Julienne Reviewed-by: Mark Rutland Reviewed-by: Paul E. McKenney ---- Changes since v3: - s/nointr/noinstr/ Changes since v2: - No big content changes, just style corrections, so it should be pretty clean at this stage. In the light of this, I kept Mark's Reviewed-by. - Paul's style and paragraph re-writes - Randy's style comments - Add links to transition type sections Documentation/core-api/entry.rst | 261 +++++++++++++++++++++++++++++++ Documentation/core-api/index.rst | 8 + 2 files changed, 269 insertions(+) create mode 100644 Documentation/core-api/entry.rst Reviewed-by: Frederic Weisbecker Link: https://lore.kernel.org/r/20220110105044.94423-1-nsaenzju@redhat.com Signed-off-by: Jonathan Corbet --- Documentation/core-api/entry.rst | 261 +++++++++++++++++++++++++++++++++++++++ Documentation/core-api/index.rst | 8 ++ 2 files changed, 269 insertions(+) create mode 100644 Documentation/core-api/entry.rst (limited to 'Documentation/core-api') diff --git a/Documentation/core-api/entry.rst b/Documentation/core-api/entry.rst new file mode 100644 index 000000000000..c6f8e22c88fe --- /dev/null +++ b/Documentation/core-api/entry.rst @@ -0,0 +1,261 @@ +Entry/exit handling for exceptions, interrupts, syscalls and KVM +================================================================ + +All transitions between execution domains require state updates which are +subject to strict ordering constraints. State updates are required for the +following: + + * Lockdep + * RCU / Context tracking + * Preemption counter + * Tracing + * Time accounting + +The update order depends on the transition type and is explained below in +the transition type sections: `Syscalls`_, `KVM`_, `Interrupts and regular +exceptions`_, `NMI and NMI-like exceptions`_. + +Non-instrumentable code - noinstr +--------------------------------- + +Most instrumentation facilities depend on RCU, so intrumentation is prohibited +for entry code before RCU starts watching and exit code after RCU stops +watching. In addition, many architectures must save and restore register state, +which means that (for example) a breakpoint in the breakpoint entry code would +overwrite the debug registers of the initial breakpoint. + +Such code must be marked with the 'noinstr' attribute, placing that code into a +special section inaccessible to instrumentation and debug facilities. Some +functions are partially instrumentable, which is handled by marking them +noinstr and using instrumentation_begin() and instrumentation_end() to flag the +instrumentable ranges of code: + +.. code-block:: c + + noinstr void entry(void) + { + handle_entry(); // <-- must be 'noinstr' or '__always_inline' + ... + + instrumentation_begin(); + handle_context(); // <-- instrumentable code + instrumentation_end(); + + ... + handle_exit(); // <-- must be 'noinstr' or '__always_inline' + } + +This allows verification of the 'noinstr' restrictions via objtool on +supported architectures. + +Invoking non-instrumentable functions from instrumentable context has no +restrictions and is useful to protect e.g. state switching which would +cause malfunction if instrumented. + +All non-instrumentable entry/exit code sections before and after the RCU +state transitions must run with interrupts disabled. + +Syscalls +-------- + +Syscall-entry code starts in assembly code and calls out into low-level C code +after establishing low-level architecture-specific state and stack frames. This +low-level C code must not be instrumented. A typical syscall handling function +invoked from low-level assembly code looks like this: + +.. code-block:: c + + noinstr void syscall(struct pt_regs *regs, int nr) + { + arch_syscall_enter(regs); + nr = syscall_enter_from_user_mode(regs, nr); + + instrumentation_begin(); + if (!invoke_syscall(regs, nr) && nr != -1) + result_reg(regs) = __sys_ni_syscall(regs); + instrumentation_end(); + + syscall_exit_to_user_mode(regs); + } + +syscall_enter_from_user_mode() first invokes enter_from_user_mode() which +establishes state in the following order: + + * Lockdep + * RCU / Context tracking + * Tracing + +and then invokes the various entry work functions like ptrace, seccomp, audit, +syscall tracing, etc. After all that is done, the instrumentable invoke_syscall +function can be invoked. The instrumentable code section then ends, after which +syscall_exit_to_user_mode() is invoked. + +syscall_exit_to_user_mode() handles all work which needs to be done before +returning to user space like tracing, audit, signals, task work etc. After +that it invokes exit_to_user_mode() which again handles the state +transition in the reverse order: + + * Tracing + * RCU / Context tracking + * Lockdep + +syscall_enter_from_user_mode() and syscall_exit_to_user_mode() are also +available as fine grained subfunctions in cases where the architecture code +has to do extra work between the various steps. In such cases it has to +ensure that enter_from_user_mode() is called first on entry and +exit_to_user_mode() is called last on exit. + + +KVM +--- + +Entering or exiting guest mode is very similar to syscalls. From the host +kernel point of view the CPU goes off into user space when entering the +guest and returns to the kernel on exit. + +kvm_guest_enter_irqoff() is a KVM-specific variant of exit_to_user_mode() +and kvm_guest_exit_irqoff() is the KVM variant of enter_from_user_mode(). +The state operations have the same ordering. + +Task work handling is done separately for guest at the boundary of the +vcpu_run() loop via xfer_to_guest_mode_handle_work() which is a subset of +the work handled on return to user space. + +Interrupts and regular exceptions +--------------------------------- + +Interrupts entry and exit handling is slightly more complex than syscalls +and KVM transitions. + +If an interrupt is raised while the CPU executes in user space, the entry +and exit handling is exactly the same as for syscalls. + +If the interrupt is raised while the CPU executes in kernel space the entry and +exit handling is slightly different. RCU state is only updated when the +interrupt is raised in the context of the CPU's idle task. Otherwise, RCU will +already be watching. Lockdep and tracing have to be updated unconditionally. + +irqentry_enter() and irqentry_exit() provide the implementation for this. + +The architecture-specific part looks similar to syscall handling: + +.. code-block:: c + + noinstr void interrupt(struct pt_regs *regs, int nr) + { + arch_interrupt_enter(regs); + state = irqentry_enter(regs); + + instrumentation_begin(); + + irq_enter_rcu(); + invoke_irq_handler(regs, nr); + irq_exit_rcu(); + + instrumentation_end(); + + irqentry_exit(regs, state); + } + +Note that the invocation of the actual interrupt handler is within a +irq_enter_rcu() and irq_exit_rcu() pair. + +irq_enter_rcu() updates the preemption count which makes in_hardirq() +return true, handles NOHZ tick state and interrupt time accounting. This +means that up to the point where irq_enter_rcu() is invoked in_hardirq() +returns false. + +irq_exit_rcu() handles interrupt time accounting, undoes the preemption +count update and eventually handles soft interrupts and NOHZ tick state. + +In theory, the preemption count could be updated in irqentry_enter(). In +practice, deferring this update to irq_enter_rcu() allows the preemption-count +code to be traced, while also maintaining symmetry with irq_exit_rcu() and +irqentry_exit(), which are described in the next paragraph. The only downside +is that the early entry code up to irq_enter_rcu() must be aware that the +preemption count has not yet been updated with the HARDIRQ_OFFSET state. + +Note that irq_exit_rcu() must remove HARDIRQ_OFFSET from the preemption count +before it handles soft interrupts, whose handlers must run in BH context rather +than irq-disabled context. In addition, irqentry_exit() might schedule, which +also requires that HARDIRQ_OFFSET has been removed from the preemption count. + +NMI and NMI-like exceptions +--------------------------- + +NMIs and NMI-like exceptions (machine checks, double faults, debug +interrupts, etc.) can hit any context and must be extra careful with +the state. + +State changes for debug exceptions and machine-check exceptions depend on +whether these exceptions happened in user-space (breakpoints or watchpoints) or +in kernel mode (code patching). From user-space, they are treated like +interrupts, while from kernel mode they are treated like NMIs. + +NMIs and other NMI-like exceptions handle state transitions without +distinguishing between user-mode and kernel-mode origin. + +The state update on entry is handled in irqentry_nmi_enter() which updates +state in the following order: + + * Preemption counter + * Lockdep + * RCU / Context tracking + * Tracing + +The exit counterpart irqentry_nmi_exit() does the reverse operation in the +reverse order. + +Note that the update of the preemption counter has to be the first +operation on enter and the last operation on exit. The reason is that both +lockdep and RCU rely on in_nmi() returning true in this case. The +preemption count modification in the NMI entry/exit case must not be +traced. + +Architecture-specific code looks like this: + +.. code-block:: c + + noinstr void nmi(struct pt_regs *regs) + { + arch_nmi_enter(regs); + state = irqentry_nmi_enter(regs); + + instrumentation_begin(); + nmi_handler(regs); + instrumentation_end(); + + irqentry_nmi_exit(regs); + } + +and for e.g. a debug exception it can look like this: + +.. code-block:: c + + noinstr void debug(struct pt_regs *regs) + { + arch_nmi_enter(regs); + + debug_regs = save_debug_regs(); + + if (user_mode(regs)) { + state = irqentry_enter(regs); + + instrumentation_begin(); + user_mode_debug_handler(regs, debug_regs); + instrumentation_end(); + + irqentry_exit(regs, state); + } else { + state = irqentry_nmi_enter(regs); + + instrumentation_begin(); + kernel_mode_debug_handler(regs, debug_regs); + instrumentation_end(); + + irqentry_nmi_exit(regs, state); + } + } + +There is no combined irqentry_nmi_if_kernel() function available as the +above cannot be handled in an exception-agnostic way. diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst index 5de2c7a4b1b3..972d46a5ddf6 100644 --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -44,6 +44,14 @@ Library functionality that is used throughout the kernel. timekeeping errseq +Low level entry and exit +======================== + +.. toctree:: + :maxdepth: 1 + + entry + Concurrency primitives ====================== -- cgit v1.2.3