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 <tglx@linutronix.de
Co-developed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

----

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 <frederic@kernel.org>
Link: https://lore.kernel.org/r/20220110105044.94423-1-nsaenzju@redhat.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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.

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
 ======================