[PATCH 9/9] bpf, docs: Add note about reserved instruction

[dthaler1968@googlemail.com]

From: Dave Thaler <dthaler@microsoft.com>

Add note about reserved instruction

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
---
 Documentation/bpf/clang-notes.rst     | 5 +++++
 Documentation/bpf/instruction-set.rst | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/Documentation/bpf/clang-notes.rst b/Documentation/bpf/clang-notes.rst
index 528feddf2..40c618551 100644
--- a/Documentation/bpf/clang-notes.rst
+++ b/Documentation/bpf/clang-notes.rst
@@ -20,6 +20,11 @@ Arithmetic instructions
 For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with
 ``-Xclang -target-feature -Xclang +alu32``.  In CPU version 3, support is automatically included.
 
+Reserved instructions
+====================
+
+Clang will generate the reserved ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d) instruction if ``-O0`` is used.
+
 Atomic operations
 =================
 
diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index d0685a06f..72d59b3c6 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -301,6 +301,9 @@ with the remaining registers being ignored.  The definition of a helper function
 is responsible for specifying the type (e.g., integer, pointer, etc.) of the value returned,
 the number of arguments, and the type of each argument.
 
+Note that ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d), where the helper function integer
+would be read from a specified register, is reserved and currently not permitted.
+
 Runtime functions
 ~~~~~~~~~~~~~~~~~
 Runtime functions are like helper functions except that they are not specific
-- 
2.33.4