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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>
This commit is contained in:
Brendan Jackman 2025-10-15 17:02:05 +00:00 committed by Jonathan Corbet
parent ba2457109d
commit aab703b3c6
1 changed files with 5 additions and 1 deletions
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6
Documentation/userspace-api/spec_ctrl.rst
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@ -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);