From: Brendan Jackman Date: Wed, 15 Oct 2025 17:02:05 +0000 (+0000) Subject: Documentation: clarify PR_SPEC_L1D_FLUSH X-Git-Tag: v6.19-rc1~184^2~49 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=aab703b3c6f62f698d294444abe05cc83b50cff2;p=thirdparty%2Flinux.git 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 Signed-off-by: Jonathan Corbet 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 5e8ed9eef9aa8..ca89151fc0a8e 100644 --- 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);