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Date:   Sun, 10 May 2020 11:48:03 +1000
From:   Balbir Singh <>
To:     <>, <>
CC:     <>, <>,
        <>, <>,
        <>, <>,
        <>, Balbir Singh <>
Subject: [PATCH v6 6/6] Documentation: Add L1D flushing Documentation

Add documentation of l1d flushing, explain the need for the
feature and how it can be used.

Signed-off-by: Balbir Singh <>
Reviewed-by: Kees Cook <>
 Documentation/admin-guide/hw-vuln/index.rst   |  1 +
 .../admin-guide/hw-vuln/l1d_flush.rst         | 40 +++++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100644 Documentation/admin-guide/hw-vuln/l1d_flush.rst

diff --git a/Documentation/admin-guide/hw-vuln/index.rst b/Documentation/admin-guide/hw-vuln/index.rst
index 0795e3c2643f..35633b299d45 100644
--- a/Documentation/admin-guide/hw-vuln/index.rst
+++ b/Documentation/admin-guide/hw-vuln/index.rst
@@ -14,3 +14,4 @@ are configurable at compile, boot or run time.
+   l1d_flush
diff --git a/Documentation/admin-guide/hw-vuln/l1d_flush.rst b/Documentation/admin-guide/hw-vuln/l1d_flush.rst
new file mode 100644
index 000000000000..7d515b8c29f1
--- /dev/null
+++ b/Documentation/admin-guide/hw-vuln/l1d_flush.rst
@@ -0,0 +1,40 @@
+L1D Flushing for the paranoid
+With an increasing number of vulnerabilities being reported around data
+leaks from L1D, a new user space mechanism to flush the L1D cache on
+context switch is added to the kernel. This should help address
+CVE-2020-0550 and for paranoid applications, keep them safe from any
+yet to be discovered vulnerabilities, related to leaks from the L1D
+Tasks can opt in to this mechanism by using a prctl (implemented only
+for x86 at the moment).
+Related CVES
+At the present moment, the following CVEs can be addressed by this
+    =============       ========================     ==================
+    CVE-2020-0550       Improper Data Forwarding     OS related aspects
+    =============       ========================     ==================
+Usage Guidelines
+Applications can call ``prctl(2)`` with one of these two arguments
+1. PR_SET_L1D_FLUSH - flush the L1D cache on context switch (out)
+2. PR_GET_L1D_FLUSH - get the current state of the L1D cache flush, returns 1
+   if set and 0 if not set.
+**NOTE**: The feature is disabled by default, applications to need to specifically
+opt into the feature to enable it.
+When PR_SET_L1D_FLUSH is enabled for a task, on switching tasks (when
+the address space changes), a flush of the L1D cache is performed for
+the task when it leaves the CPU. If the underlying CPU supports L1D
+flushing in hardware, the hardware mechanism is used, otherwise a software
+fallback, similar to the mechanism used by L1TF is used.

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