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authorNicolas Saenz Julienne <nsaenzju@redhat.com>2022-01-10 11:50:44 +0100
committerJonathan Corbet <corbet@lwn.net>2022-01-27 11:32:40 -0700
commite3aa43e936d854373d9a75372aefcefebfca208f (patch)
tree3fecc7e74fc3a694567d4d54907b40895091b22d /Documentation/core-api
parentbf026e2e316ba57135b70e8ce591276239c7b2cf (diff)
downloadlinux-e3aa43e936d854373d9a75372aefcefebfca208f.tar.bz2
Documentation: core-api: entry: Add comments about nesting
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> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@kernel.org> Link: https://lore.kernel.org/r/20220110105044.94423-2-nsaenzju@redhat.com Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation/core-api')
-rw-r--r--Documentation/core-api/entry.rst18
1 files changed, 18 insertions, 0 deletions
diff --git a/Documentation/core-api/entry.rst b/Documentation/core-api/entry.rst
index c6f8e22c88fe..e12f22ab33c7 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.
+Do not nest syscalls. Nested systcalls will cause RCU and/or context tracking
+to print a warning.
KVM
---
@@ -121,6 +123,8 @@ 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.
+Do not nest KVM entry/exit transitions because doing so is nonsensical.
+
Interrupts and regular exceptions
---------------------------------
@@ -180,6 +184,16 @@ 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 +273,7 @@ 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.