[Minios-devel] [UNIKRAFT PATCH 6/6] doc: Document reserved `isr` variant

[Simon Kuenzer <simon.kuenzer@xxxxxxxxx>]

Document the reserved `isr` variant.

Signed-off-by: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>
---
 doc/guides/developers-app.rst | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/doc/guides/developers-app.rst b/doc/guides/developers-app.rst
index 5b5f0918..9adce4dd 100644
--- a/doc/guides/developers-app.rst
+++ b/doc/guides/developers-app.rst
@@ -220,12 +220,21 @@ syntax: ::
   APPNAME_FILENAME_INCLUDES-y += -Iextra/include
 
 It is also be possible compile a single source files multiple times with
-different flags. For this case, Unikore supports variants:
+different flags. For this case, Unikraft supports variants: ::
 
   APPNAME_SRCS-y += $(APPNAME_BASE)/filename.c|variantname
   APPNAME_FILENAME_VARIANTNAME_FLAGS-y += -DFLAG2
   APPNAME_FILENAME_VARIANTNAME_INCLUDES-y += -Iextra/include
 
+.. note:: The build system treats the reserved ``isr`` variant specially:
+         This variant is intended for build units that contain code that can
+         be called from interrupt context, as well. Different global
+         architecture flags are used to generate interrupt-safe code
+         (``ISR_ARCHFLAGS-y`` instead of ``ARCHFLAGS-y``). Generally, these
+         flags avoid using extended machine units which aren't saved by the
+         processor before entering interrupt context (e.g., floating point
+         units, vector units).
+
 Finally, you may also need to provide "glue" code, for instance to
 implement the ``main()`` function that Unikraft expects you to
 implement by calling your application's main or init routines. As a
-- 
2.20.1


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