arm64: mte: Add Memory Tagging Extension documentation

Memory Tagging Extension (part of the ARMv8.5 Extensions) provides
a mechanism to detect the sources of memory related errors which
may be vulnerable to exploitation, including bounds violations,
use-after-free, use-after-return, use-out-of-scope and use before
initialization errors.

Add Memory Tagging Extension documentation for the arm64 linux
kernel support.
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Co-developed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: Will Deacon <will@kernel.org>

Documentation/arm64/cpu-feature-registers.rst

@@ -175,6 +175,8 @@ infrastructure:
      +------------------------------+---------+---------+
      | Name                         |  bits   | visible |
      +------------------------------+---------+---------+
+     | MTE                          | [11-8]  |    y    |
+     +------------------------------+---------+---------+
      | SSBS                         | [7-4]   |    y    |
      +------------------------------+---------+---------+
      | BT                           | [3-0]   |    y    |

Documentation/arm64/elf_hwcaps.rst

@@ -240,6 +240,10 @@ HWCAP2_BTI
 
     Functionality implied by ID_AA64PFR0_EL1.BT == 0b0001.
 
+HWCAP2_MTE
+
+    Functionality implied by ID_AA64PFR1_EL1.MTE == 0b0010, as described
+    by Documentation/arm64/memory-tagging-extension.rst.
 
 4. Unused AT_HWCAP bits
 -----------------------

Documentation/arm64/index.rst

@@ -14,6 +14,7 @@ ARM64 Architecture
     hugetlbpage
     legacy_instructions
     memory
+    memory-tagging-extension
     perf
     pointer-authentication
     silicon-errata