feat(el3-runtime): handle traps for IMPDEF registers accesses

This patch introduces support to handle traps from lower ELs for
IMPDEF system register accesses. The actual support is left to the
platforms to implement.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
Change-Id: I623d5c432b4ce4328b68f238c15b1c83df97c1e5

Makefile

@@ -1204,6 +1204,7 @@ $(eval $(call assert_numerics,\
         TWED_DELAY \
         ENABLE_FEAT_TWED \
         SVE_VECTOR_LEN \
+	IMPDEF_SYSREG_TRAP \
 )))
 
 ifdef KEY_SIZE
@@ -1333,6 +1334,7 @@ $(eval $(call add_defines,\
         TWED_DELAY \
         ENABLE_FEAT_TWED \
 	CONDITIONAL_CMO \
+	IMPDEF_SYSREG_TRAP \
 )))
 
 ifeq (${SANITIZE_UB},trap)

bl31/bl31_traps.c

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2022, ARM Limited. All rights reserved.
+ * Copyright (c) 2023, NVIDIA Corporation. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *
@@ -11,13 +12,19 @@
 
 int handle_sysreg_trap(uint64_t esr_el3, cpu_context_t *ctx)
 {
-	switch (esr_el3 & ISS_SYSREG_OPCODE_MASK) {
+	uint64_t __unused opcode = esr_el3 & ISS_SYSREG_OPCODE_MASK;
+
 #if ENABLE_FEAT_RNG_TRAP
-	case ISS_SYSREG_OPCODE_RNDR:
-	case ISS_SYSREG_OPCODE_RNDRRS:
+	if ((opcode == ISS_SYSREG_OPCODE_RNDR) || (opcode == ISS_SYSREG_OPCODE_RNDRRS)) {
 		return plat_handle_rng_trap(esr_el3, ctx);
-#endif
-	default:
-		return TRAP_RET_UNHANDLED;
 	}
+#endif
+
+#if IMPDEF_SYSREG_TRAP
+	if ((opcode & ISS_SYSREG_OPCODE_IMPDEF) == ISS_SYSREG_OPCODE_IMPDEF) {
+		return plat_handle_impdef_trap(esr_el3, ctx);
+	}
+#endif
+
+	return TRAP_RET_UNHANDLED;
 }

docs/getting_started/build-options.rst

@@ -627,6 +627,10 @@ Common build options
    translation library (xlat tables v2) must be used; version 1 of translation
    library is not supported.
 
+-  ``IMPDEF_SYSREG_TRAP``: Numeric value to enable the handling traps for
+   implementation defined system register accesses from lower ELs. Default
+   value is ``0``.
+
 -  ``INVERTED_MEMMAP``: memmap tool print by default lower addresses at the
    bottom, higher addresses at the top. This build flag can be set to '1' to
    invert this behavior. Lower addresses will be printed at the top and higher

docs/porting-guide.rst

@@ -3530,6 +3530,38 @@ The return value indicates how to proceed:
 
 This function needs to be implemented by a platform if it enables FEAT_RNG_TRAP.
 
+Function : plat_handle_impdef_trap
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : uint64_t
+    Argument : cpu_context_t *
+    Return   : int
+
+This function is invoked by BL31's exception handler when there is a synchronous
+system register trap caused by access to the implementation defined registers.
+It allows platforms enabling ``IMPDEF_SYSREG_TRAP`` to emulate those system
+registers choosing to program bits of their choice.
+
+The first parameter (``uint64_t esr_el3``) contains the content of the ESR_EL3
+syndrome register, which encodes the instruction that was trapped.
+
+The second parameter (``cpu_context_t *ctx``) represents the CPU state in the
+lower exception level, at the time when the execution of the ``mrs`` instruction
+was trapped.
+
+The return value indicates how to proceed:
+
+-  When returning ``TRAP_RET_UNHANDLED`` (-1), the machine will panic.
+-  When returning ``TRAP_RET_REPEAT`` (0), the exception handler will return
+   to the same instruction, so its execution will be repeated.
+-  When returning ``TRAP_RET_CONTINUE`` (1), the exception handler will return
+   to the next instruction.
+
+This function needs to be implemented by a platform if it enables
+IMPDEF_SYSREG_TRAP.
+
 Build flags
 -----------
 

include/bl31/sync_handle.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2022, ARM Limited. All rights reserved.
+ * Copyright (c) 2023, NVIDIA Corporation. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -16,6 +17,7 @@
 #define ISS_SYSREG_DIRECTION_MASK	0x000001UL
 
 #define ISS_SYSREG_OPCODE_RNDR		0x30c808U
+#define ISS_SYSREG_OPCODE_IMPDEF	0x303c00U
 #define ISS_SYSREG_OPCODE_RNDRRS	0x32c808U
 
 #define TRAP_RET_UNHANDLED		-1
@@ -54,6 +56,7 @@ static inline bool is_sysreg_iss_write(uint64_t esr)
 int handle_sysreg_trap(uint64_t esr_el3, cpu_context_t *ctx);
 
 /* Prototypes for system register emulation handlers provided by platforms. */
+int plat_handle_impdef_trap(uint64_t esr_el3, cpu_context_t *ctx);
 int plat_handle_rng_trap(uint64_t esr_el3, cpu_context_t *ctx);
 
 #endif /* __ASSEMBLER__ */

make_helpers/defaults.mk

@@ -240,6 +240,9 @@ HASH_ALG			:= sha256
 # operations.
 HW_ASSISTED_COHERENCY		:= 0
 
+# Flag to enable trapping of implementation defined sytem registers
+IMPDEF_SYSREG_TRAP		:= 0
+
 # Set the default algorithm for the generation of Trusted Board Boot keys
 KEY_ALG				:= rsa