[PATCH] arm64: Document requirements for fine grained traps at boot

[Mark Brown]

The arm64 FEAT_FGT extension introduces a set of traps to EL2 for accesses
to small sets of registers and instructions from EL1 and EL0.  Currently
Linux makes no use of this feature, explicitly document that it should
be disabled when entering the kernel at EL2 (as is the architectural
default) to help avoid surprises.

Signed-off-by: Mark Brown <broonie at kernel.org>
---
 Documentation/arm64/booting.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/arm64/booting.rst b/Documentation/arm64/booting.rst
index 7552dbc1cc54..1efc2d3023bb 100644
--- a/Documentation/arm64/booting.rst
+++ b/Documentation/arm64/booting.rst
@@ -270,6 +270,13 @@ Before jumping into the kernel, the following conditions must be met:
       having 0b1 set for the corresponding bit for each of the auxiliary
       counters present.
 
+  For CPUs with Fine Grained Traps (FEAT_FGT) extension present:
+
+  - If the kernel is entered at EL2:
+
+    - HAFGRTR_EL2, HDFGWTR_EL2, HDFGRTR_EL2, HFGWTR_EL2, HFGRTR_EL2 and
+      HFGITR_EL2 must be initialised to 0.
+
 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.
-- 
2.20.1