Commit aab703b3 authored by Brendan Jackman's avatar Brendan Jackman Committed by Jonathan Corbet
Browse files

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: default avatarBrendan Jackman <jackmanb@google.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
Message-ID: <20251015-l1d-flush-doc-v1-1-f8cefea3f2f2@google.com>
parent ba245710

Documentation/userspace-api/spec_ctrl.rst

+5 −1
Original line number Diff line number Diff line
@@ -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);