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Subject[PATCH v2 4/4] arch/x86: 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 <>
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..73ee9e491a74
--- /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 an architecture specific
+prctl (x86 only 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 ``arch_prctl(2)`` with one of these two arguments
+1. ARCH_SET_L1D_FLUSH - flush the L1D cache on context switch (out)
+2. ARCH_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 ARCH_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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