[PATCH v3 2/2] Documentation: core-api: entry: Add comments about nesting

[Nicolas Saenz Julienne <nsaenzju@redhat.com>]

The topic of nesting and reentrancy in the context of early entry code
hasn't been addressed so far. So do it.

Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>

---

NOTE: I moved this into a separate patch to simplify the review.

 Documentation/core-api/entry.rst | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/Documentation/core-api/entry.rst b/Documentation/core-api/entry.rst
index 3f80537f2826..f665f201ead0 100644
--- a/Documentation/core-api/entry.rst
+++ b/Documentation/core-api/entry.rst
@@ -105,6 +105,8 @@ 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.
 
+Syscalls shouldn't nest. If it were to happen, RCU / context tracking will
+catch the misbehavior and print out a warning.
 
 KVM
 ---
@@ -121,6 +123,9 @@ 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.
 
+Nesting doesn't make sense in the context of KVM entry/exit transitions, it
+shouldn't happen.
+
 Interrupts and regular exceptions
 ---------------------------------
 
@@ -180,6 +185,16 @@ 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.
 
+Even though interrupt handlers are expected to run with local interrupts
+disabled, interrupt nesting is common from an entry/exit perspective. For
+example, softirq handling happens within an irqentry_{enter,exit}() block, with
+local interrupts enabled. Also, although uncommon, nothing prevents an
+interrupt handler from re-enabling interrupts.
+
+Interrupt entry/exit code doesn't strictly need to handle reentrancy, since it
+runs with local interrupts disabled. But NMIs can happen anytime, and a lot of
+the entry code is shared between the two.
+
 NMI and NMI-like exceptions
 ---------------------------
 
@@ -259,3 +274,7 @@ and for e.g. a debug exception it can look like this:
 
 There is no combined irqentry_nmi_if_kernel() function available as the
 above cannot be handled in an exception-agnostic way.
+
+NMIs can happen in any context. For example, an NMI-like exception triggered
+while handling an NMI. So NMI entry code has to be reentrant and state updates
+need to handle nesting.
-- 
2.33.1