[PATCH v2] Documentation: Fill the gaps about entry/noinstr constraints

[Thomas Gleixner]

The entry/exit handling for exceptions, interrupts, syscalls and KVM is
not really documented except for some comments.

Fill the gaps.

Reported-by: Nicolas Saenz Julienne <nsaenzju at redhat.com>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
---
V2: Be more precise, fix formatting and typos - Mark 
---
 Documentation/core-api/entry.rst |  274 +++++++++++++++++++++++++++++++++++++++
 Documentation/core-api/index.rst |    8 +
 2 files changed, 282 insertions(+)

--- /dev/null
+++ b/Documentation/core-api/entry.rst
@@ -0,0 +1,274 @@
+Entry/exit handling for exceptions, interrupts, syscalls and KVM
+================================================================
+
+For any transition from one execution domain into another the kernel
+requires update of various states. The state updates have strict rules
+versus ordering.
+
+The states which need to be updated are:
+
+  * Lockdep
+  * RCU
+  * Preemption counter
+  * Tracing
+  * Time accounting
+
+The update order depends on the transition type and is explained below in
+the transition type sections.
+
+Non-instrumentable code - noinstr
+---------------------------------
+
+Low level transition code cannot be instrumented before RCU is watching and
+after RCU went into a non watching state (NOHZ, NOHZ_FULL) as most
+instrumentation facilities depend on RCU.
+
+Aside of that many architectures have to save register state, e.g. debug or
+cause registers before another exception of the same type can happen. A
+breakpoint in the breakpoint entry code would overwrite the debug registers
+of the inital breakpoint.
+
+Such code has to be marked with the 'noinstr' attribute. That places the
+code into a special section which is taboo for instrumentation and debug
+facilities.
+
+In a function which is marked 'noinstr' it's only allowed to call into
+non-instrumentable code except when the invocation of instrumentable code
+is annotated with a instrumentation_begin()/instrumentation_end() pair:
+
+.. 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 low level architecture specific assembly code
+and calls out into C-code after establishing low level architecture
+specific state and stack frames. This low level 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 the function returns instrumentable code
+can be invoked. After returning from the syscall handler the instrumentable
+code section ends and 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 was raised in context of the CPU's idle task because that's the
+only kernel context where RCU can be not watching on NOHZ enabled kernels.
+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.
+
+The preemption count could be established in irqentry_enter() already, but
+there is no real value to do so. This allows the preemption count to be
+traced and just puts a restriction on the early entry code up to
+irq_enter_rcu().
+
+This also keeps the handling vs. irq_exit_rcu() symmetric and
+irq_exit_rcu() must undo the preempt count elevation before handling soft
+interrupts and irqentry_exit() also requires that because it might
+schedule.
+
+
+NMI and NMI-like exceptions
+---------------------------
+
+NMIs and NMI like exceptions, e.g. Machine checks, double faults, debug
+interrupts etc. can hit any context and have to be extra careful vs. the
+state.
+
+Debug exceptions can handle user space breakpoints or watchpoints in the
+same way as an interrupt which was raised while executing in user space,
+but kernel mode debug exceptions have to be treated like NMIs as they can
+even happen in NMI context, e.g. due to code patching.
+
+Also Machine check exceptions can handle user mode exceptions like regular
+interrupts, but for kernel mode exceptions they have to be treated like
+NMIs.
+
+NMIs and the other NMI-like exceptions handle state transitions in the most
+straight forward way and do not differentiate between user 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
+  * 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.
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -44,6 +44,14 @@ Library functionality that is used throu
    timekeeping
    errseq
 
+Low level entry and exit
+========================
+
+.. toctree::
+   :maxdepth: 1
+
+   entry
+
 Concurrency primitives
 ======================