Documentation: clarify PR_SPEC_L1D_FLUSH

For PR_SPEC_STORE_BYPASS and PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE
means "disable the speculation bug" i.e. "enable the mitigation".

For PR_SPEC_L1D_FLUSH, PR_SPEC_DISABLE means "disable the mitigation".
This is not obvious, so document it.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251015-l1d-flush-doc-v1-1-f8cefea3f2f2@google.com>

diff --git a/Documentation/userspace-api/spec_ctrl.rst b/Documentation/userspace-api/spec_ctrl.rst
index 5e8ed9eef9aa84f12fa255af7b15c163ff01aab8..ca89151fc0a8e7205e0a0062134d63b213b9ef11 100644 (file)
--- a/Documentation/userspace-api/spec_ctrl.rst
+++ b/Documentation/userspace-api/spec_ctrl.rst
@@ -26,7 +26,8 @@ PR_GET_SPECULATION_CTRL
 
 PR_GET_SPECULATION_CTRL returns the state of the speculation misfeature
 which is selected with arg2 of prctl(2). The return value uses bits 0-3 with
-the following meaning:
+the following meaning (with the caveat that PR_SPEC_L1D_FLUSH has less obvious
+semantics, see documentation for that specific control below):
 
 ==== ====================== ==================================================
 Bit  Define                 Description
@@ -110,6 +111,9 @@ Speculation misfeature controls
 - PR_SPEC_L1D_FLUSH: Flush L1D Cache on context switch out of the task
                         (works only when tasks run on non SMT cores)
 
+For this control, PR_SPEC_ENABLE means that the **mitigation** is enabled (L1D
+is flushed), PR_SPEC_DISABLE means it is disabled.
+
   Invocations:
    * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, 0, 0, 0);
    * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_ENABLE, 0, 0);