Hi Thomas,

On Tue, Nov 30, 2021 at 11:31:30PM +0100, Thomas Gleixner wrote:

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

Fill the gaps.

Thanks for this! Now there's less chance I'll get this wrong. :)

I have a couple of minor comments below -- mostly typo/formatting junk.

Reported-by: Nicolas Saenz Julienne <redacted>
Signed-off-by: Thomas Gleixner <redacted>
---
 Documentation/core-api/entry.rst |  268 +++++++++++++++++++++++++++++++++++++++
 Documentation/core-api/index.rst |    8 +
 kernel/entry/common.c            |    1 

I think the change to kernel/entry/common.c got included by accident?

As a small nit, can we remove the "obviously"? It's certainly obvious to you
and me, but it doesn't meaningfully affect the sentence either way.

+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::
+
+  noinstr void do_syscall(struct pt_regs \*regs, int nr)

                                            ^^

Is `\*` necessary here? ... and/or should this be an explicit code block (which
IIUC doesn't require this esacping), e.g.

.. code-block:: c

      noinstr void do_syscall(struct pt_regs *regs, int nr)
	{
		...
	}

Similar comment for the other code snippets in this patch.

+  {
+	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 idle task because that's the only

Since we have an idle task for each cpu, perhaps either:

  s/the idle task/an idle task/
  s/the idle task/the CPU's idle task/

+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::
+
+  noinstr void do_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 irqenttry_nmi_exit() does the reverse operation in the

                        ^^^^^^^^^

 s/irqenttry/irqentry/

+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 can obviously not
+be traced.

Could we say "must not" instead of "can not", e.g.

  The preemption count modification in the NMI entry/exit must not be traced.

That way it's clearly a requirement, rather than a limitation.

+Architecture specific code looks like this::
+
+  noinstr void do_nmi(struct pt_regs \*regs)
+  {
+	arch_nmi_enter(regs);
+	state = irqentry_nmi_enter(regs);
+
+	instrumentation_begin();
+
+	invoke_nmi_handler(regs);
+
+	instrumentation_end();
+	irqentry_nmi_exit(regs);
+  }

To keep the begin/end and enter/exit calls visually balanced, should the
instrumentation_end() call have trailing a line space, e.g.

e.g.

	arch_nmi_enter(regs);
	state = irqentry_nmi_enter(regs);

	instrumentation_begin();

	invoke_nmi_handler(regs);

	instrumentation_end();

	irqentry_nmi_exit(regs);

... or sandwiched around invoke_nmi_handler(), e.g.

	arch_nmi_enter(regs);
	state = irqentry_nmi_enter(regs);

	instrumentation_begin();
	invoke_nmi_handler(regs);
	instrumentation_end();

	irqentry_nmi_exit(regs);

Since the examples in this file are fairly simple I'd suggest the latter.

Unrelated whitespace change?

Thanks,
Mark.

 #include <linux/context_tracking.h>
 #include <linux/entry-common.h>
 #include <linux/highmem.h>

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