[PATCH v10 04/40] arm64: Document boot requirements for Guarded Control Stacks

[Mark Brown]

FEAT_GCS introduces a number of new system registers, we require that
access to these registers is not trapped when we identify that the feature
is detected.  Since if GCS is enabled any function call instruction will
be checked we also require that the feature be specifically disabled.

Signed-off-by: Mark Brown <broonie at kernel.org>
---
 Documentation/arch/arm64/booting.rst | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/Documentation/arch/arm64/booting.rst b/Documentation/arch/arm64/booting.rst
index b57776a68f15..f5b8e4bb9653 100644
--- a/Documentation/arch/arm64/booting.rst
+++ b/Documentation/arch/arm64/booting.rst
@@ -411,6 +411,36 @@ Before jumping into the kernel, the following conditions must be met:
 
     - HFGRWR_EL2.nPIRE0_EL1 (bit 57) must be initialised to 0b1.
 
+ - For CPUs with Guarded Control Stacks (FEAT_GCS):
+
+  - If EL3 is present:
+
+    - SCR_EL3.GCSEn (bit 39) must be initialised to 0b1.
+
+  - If EL2 is present:
+
+    - GCSCR_EL2 must be initialised to 0.
+
+ - If the kernel is entered at EL1 and EL2 is present:
+
+    - GCSCR_EL1 must be initialised to 0.
+
+    - GCSCRE0_EL1 must be initialised to 0.
+
+    - HFGITR_EL2.nGCSEPP (bit 59) must be initialised to 0b1.
+
+    - HFGITR_EL2.nGCSSTR_EL1 (bit 58) must be initialised to 0b1.
+
+    - HFGITR_EL2.nGCSPUSHM_EL1 (bit 57) must be initialised to 0b1.
+
+    - HFGRTR_EL2.nGCS_EL1 (bit 53) must be initialised to 0b1.
+
+    - HFGRTR_EL2.nGCS_EL0 (bit 52) must be initialised to 0b1.
+
+    - HFGWTR_EL2.nGCS_EL1 (bit 53) must be initialised to 0b1.
+
+    - HFGWTR_EL2.nGCS_EL0 (bit 52) must be initialised to 0b1.
+
 The requirements described above for CPU mode, caches, MMUs, architected
 timers, coherency and system registers apply to all CPUs.  All CPUs must
 enter the kernel in the same exception level.  Where the values documented

-- 
2.39.2