refactor(twed): improve TWED enablement in EL-3

The current implementation uses plat_arm API under generic code.
"plat_arm" API is a convention used with Arm common platform layer
and is reserved for that purpose. In addition, the function has a
weak definition which is not encouraged in TF-A.

Henceforth, removing the weak API with a configurable macro "TWED_DELAY"
of numeric data type in generic code and simplifying the implementation.
By default "TWED_DELAY" is defined to zero, and the delay value need to
be explicitly set by the platforms during buildtime.

Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
Change-Id: I25cd6f628e863dc40415ced3a82d0662fdf2d75a

Makefile

@@ -1061,6 +1061,8 @@ $(eval $(call assert_numerics,\
         NR_OF_FW_BANKS \
         NR_OF_IMAGES_IN_FW_BANK \
         RAS_EXTENSION \
+        TWED_DELAY \
+        ENABLE_FEAT_TWED \
 )))
 
 ifdef KEY_SIZE
@@ -1176,6 +1178,8 @@ $(eval $(call add_defines,\
         ENABLE_FEAT_CSV2_2 \
         ENABLE_FEAT_PAN \
         FEATURE_DETECTION \
+        TWED_DELAY \
+        ENABLE_FEAT_TWED \
 )))
 
 ifeq (${SANITIZE_UB},trap)

common/feat_detect.c

@@ -203,6 +203,16 @@ static void read_feat_ecv(void)
 #endif
 }
 
+/***********************************************************
+ * Feature : FEAT_TWED (Delayed Trapping of WFE Instruction)
+ **********************************************************/
+static void read_feat_twed(void)
+{
+#if (ENABLE_FEAT_TWED == FEAT_STATE_1)
+	feat_detect_panic(is_armv8_6_twed_present(), "TWED");
+#endif
+}
+
 /******************************************************************
  * Feature : FEAT_HCX (Extended Hypervisor Configuration Register)
  *****************************************************************/
@@ -279,6 +289,7 @@ void detect_arch_features(void)
 	read_feat_amuv1p1();
 	read_feat_fgt();
 	read_feat_ecv();
+	read_feat_twed();
 
 	/* v8.7 features */
 	read_feat_hcx();

docs/getting_started/build-options.rst

@@ -327,6 +327,14 @@ Common build options
    This flag can take values 0 to 2, to align with the ``FEATURE_DETECTION``
    mechanism. Default is ``0``.
 
+-  ``ENABLE_FEAT_TWED``: Numeric value to enable the ``FEAT_TWED`` (Delayed
+   trapping of WFE Instruction) extension. ``FEAT_TWED`` is a optional feature
+   available on Arm v8.6. This flag can take values 0 to 2, to align with the
+   ``FEATURE_DETECTION`` mechanism. Default is ``0``.
+
+    When ``ENABLE_FEAT_TWED`` is set to ``1``, WFE instruction trapping gets
+    delayed by the amount of value in ``TWED_DELAY``.
+
 -  ``ENABLE_FEAT_VHE``: Numeric value to enable the ``FEAT_VHE`` (Virtualization
    Host Extensions) extension. It allows access to CONTEXTIDR_EL2 register
    during EL2 context save/restore operations.``FEAT_VHE`` is a mandatory
@@ -838,6 +846,12 @@ Common build options
       When ``EL3_EXCEPTION_HANDLING`` is ``1``, ``TSP_NS_INTR_ASYNC_PREEMPT``
       must also be set to ``1``.
 
+-  ``TWED_DELAY``: Numeric value to be set in order to delay the trapping of
+   WFE instruction. ``ENABLE_FEAT_TWED`` build option must be enabled to set
+   this delay. It can take values in the range (0-15). Default value is ``0``
+   and based on this value, 2^(TWED_DELAY + 8) cycles will be delayed.
+   Platforms need to explicitly update this value based on their requirements.
+
 -  ``USE_ARM_LINK``: This flag determines whether to enable support for ARM
    linker. When the ``LINKER`` build variable points to the armlink linker,
    this flag is enabled automatically. To enable support for armlink, platforms

docs/getting_started/porting-guide.rst

@@ -2118,21 +2118,6 @@ frequency for the CPU's generic timer. This value will be programmed into the
 of the system counter, which is retrieved from the first entry in the frequency
 modes table.
 
-Function : plat_arm_set_twedel_scr_el3() [optional]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-::
-
-    Argument : void
-    Return   : uint32_t
-
-This function is used in v8.6+ systems to set the WFE trap delay value in
-SCR_EL3. If this function returns TWED_DISABLED or is left unimplemented, this
-feature is not enabled.  The only hook provided is to set the TWED fields in
-SCR_EL3, there are similar fields in HCR_EL2, SCTLR_EL2, and SCTLR_EL1 to adjust
-the WFE trap delays in lower ELs and these fields should be set by the
-appropriate EL2 or EL1 code depending on the platform configuration.
-
 #define : PLAT_PERCPU_BAKERY_LOCK_SIZE [optional]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

include/lib/extensions/twed.h

@@ -1,16 +0,0 @@
-/*
- * Copyright (c) 2020, Arm Limited. All rights reserved.
- *
- * SPDX-License-Identifier: BSD-3-Clause
- */
-
-#ifndef TWED_H
-#define TWED_H
-
-#include <stdint.h>
-
-#define TWED_DISABLED U(0xFFFFFFFF)
-
-uint32_t plat_arm_set_twedel_scr_el3(void);
-
-#endif /* TWEDE_H */

lib/el3_runtime/aarch64/context_mgmt.c

@@ -27,11 +27,14 @@
 #include <lib/extensions/sys_reg_trace.h>
 #include <lib/extensions/trbe.h>
 #include <lib/extensions/trf.h>
-#include <lib/extensions/twed.h>
 #include <lib/utils.h>
 
-static void manage_extensions_secure(cpu_context_t *ctx);
+#if ENABLE_FEAT_TWED
+/* Make sure delay value fits within the range(0-15) */
+CASSERT(((TWED_DELAY & ~SCR_TWEDEL_MASK) == 0U), assert_twed_delay_value_check);
+#endif /* ENABLE_FEAT_TWED */
 
+static void manage_extensions_secure(cpu_context_t *ctx);
 /******************************************************************************
  * This function performs initializations that are specific to SECURE state
  * and updates the cpu context specified by 'ctx'.
@@ -329,23 +332,16 @@ static void setup_context_common(cpu_context_t *ctx, const entry_point_info_t *e
 	sctlr_elx |= SCTLR_IESB_BIT;
 #endif
 
+#if ENABLE_FEAT_TWED
 	/* Enable WFE trap delay in SCR_EL3 if supported and configured */
-	if (is_armv8_6_twed_present()) {
+	/* Set delay in SCR_EL3 */
-		uint32_t delay = plat_arm_set_twedel_scr_el3();
+	scr_el3 &= ~(SCR_TWEDEL_MASK << SCR_TWEDEL_SHIFT);
+	scr_el3 |= ((TWED_DELAY & SCR_TWEDEL_MASK)
+			<< SCR_TWEDEL_SHIFT);
 
-		if (delay != TWED_DISABLED) {
+	/* Enable WFE delay */
-			/* Make sure delay value fits */
+	scr_el3 |= SCR_TWEDEn_BIT;
-			assert((delay & ~SCR_TWEDEL_MASK) == 0U);
+#endif /* ENABLE_FEAT_TWED */
-
-			/* Set delay in SCR_EL3 */
-			scr_el3 &= ~(SCR_TWEDEL_MASK << SCR_TWEDEL_SHIFT);
-			scr_el3 |= ((delay & SCR_TWEDEL_MASK)
-					<< SCR_TWEDEL_SHIFT);
-
-			/* Enable WFE delay */
-			scr_el3 |= SCR_TWEDEn_BIT;
-		}
-	}
 
 	/*
 	 * Store the initialised SCTLR_EL1 value in the cpu_context - SCTLR_EL2

make_helpers/defaults.mk

@@ -169,6 +169,9 @@ ENABLE_FEAT_SEL2		:= 0
 # Flag to enable Virtualization Host Extensions
 ENABLE_FEAT_VHE 		:= 0
 
+# Flag to enable delayed trapping of WFE instruction (FEAT_TWED)
+ENABLE_FEAT_TWED		:= 0
+
 # By default BL31 encryption disabled
 ENCRYPT_BL31			:= 0
 
@@ -440,3 +443,9 @@ ENABLE_SYS_REG_TRACE_FOR_NS	:= 0
 # lower ELs, i.e. NS-EL2, or NS-EL1 if NS-EL2 implemented but unused
 # if FEAT_TRF is implemented.
 ENABLE_TRF_FOR_NS		:= 0
+
+# In v8.6+ platforms with delayed trapping of WFE being supported
+# via FEAT_TWED, this flag takes the delay value to be set in the
+# SCR_EL3.TWEDEL(4bit) field, when FEAT_TWED is implemented.
+# By default it takes 0, and need to be updated by the platforms.
+TWED_DELAY			:= 0

plat/common/aarch64/plat_common.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014-2020, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2014-2022, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -13,7 +13,6 @@
 #if RAS_EXTENSION
 #include <lib/extensions/ras.h>
 #endif
-#include <lib/extensions/twed.h>
 #include <lib/xlat_tables/xlat_mmu_helpers.h>
 #include <plat/common/platform.h>
 
@@ -23,7 +22,6 @@
  * platforms but may also be overridden by a platform if required.
  */
 #pragma weak bl31_plat_runtime_setup
-#pragma weak plat_arm_set_twedel_scr_el3
 
 #if SDEI_SUPPORT
 #pragma weak plat_sdei_handle_masked_trigger
@@ -105,16 +103,3 @@ void plat_default_ea_handler(unsigned int ea_reason, uint64_t syndrome, void *co
 #endif
 	panic();
 }
-
-/*******************************************************************************
- * In v8.6+ platforms with delayed trapping of WFE this hook sets the delay. It
- * is a weak function definition so can be overridden depending on the
- * requirements of a platform.  The only hook provided is for the TWED fields
- * in SCR_EL3, the TWED fields in HCR_EL2, SCTLR_EL2, and SCTLR_EL1 should be
- * configured as needed in lower exception levels.
- ******************************************************************************/
-
-uint32_t plat_arm_set_twedel_scr_el3(void)
-{
-	return TWED_DISABLED;
-}