Introduce locking primitives using CAS instruction

The ARMv8v.1 architecture extension has introduced support for far
atomics, which includes compare-and-swap. Compare and Swap instruction
is only available for AArch64.

Introduce build options to choose the architecture versions to target
ARM Trusted Firmware:

  - ARM_ARCH_MAJOR: selects the major version of target ARM
    Architecture. Default value is 8.

  - ARM_ARCH_MINOR: selects the minor version of target ARM
    Architecture. Default value is 0.

When:

  (ARM_ARCH_MAJOR > 8) || ((ARM_ARCH_MAJOR == 8) && (ARM_ARCH_MINOR >= 1)),

for AArch64, Compare and Swap instruction is used to implement spin
locks. Otherwise, the implementation falls back to using
load-/store-exclusive instructions.

Update user guide, and introduce a section in Firmware Design guide to
summarize support for features introduced in ARMv8 Architecture
Extensions.

Change-Id: I73096a0039502f7aef9ec6ab3ae36680da033f16
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>

1 files changed, 69 insertions, 1 deletions

diff --git a/lib/locks/exclusive/aarch64/spinlock.S b/lib/locks/exclusive/aarch64/spinlock.S
index 1ca59123..bdc9ea0f 100644
--- a/lib/locks/exclusive/aarch64/spinlock.S
+++ b/lib/locks/exclusive/aarch64/spinlock.S
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013-2016, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2017, ARM Limited and Contributors. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -33,7 +33,66 @@
 	.globl	spin_lock
 	.globl	spin_unlock
 
+#if (ARM_ARCH_MAJOR > 8) || ((ARM_ARCH_MAJOR == 8) && (ARM_ARCH_MINOR >= 1))
 
+/*
+ * When compiled for ARMv8.1 or later, choose spin locks based on Compare and
+ * Swap instruction.
+ */
+# define USE_CAS	1
+
+/*
+ * Lock contenders using CAS, upon failing to acquire the lock, wait with the
+ * monitor in open state. Therefore, a normal store upon unlocking won't
+ * generate an SEV. Use explicit SEV instruction with CAS unlock.
+ */
+# define COND_SEV()	sev
+
+#else
+
+# define USE_CAS	0
+
+/*
+ * Lock contenders using exclusive pairs, upon failing to acquire the lock, wait
+ * with the monitor in exclusive state. A normal store upon unlocking will
+ * implicitly generate an envent; so, no explicit SEV with unlock is required.
+ */
+# define COND_SEV()
+
+#endif
+
+#if USE_CAS
+
+	.arch	armv8.1-a
+
+/*
+ * Acquire lock using Compare and Swap instruction.
+ *
+ * Compare for 0 with acquire semantics, and swap 1. Wait until CAS returns
+ * 0.
+ *
+ * void spin_lock(spinlock_t *lock);
+ */
+func spin_lock
+	mov	w2, #1
+	sevl
+1:
+	wfe
+	mov	w1, wzr
+	casa	w1, w2, [x0]
+	cbnz	w1, 1b
+	ret
+endfunc spin_lock
+
+	.arch	armv8-a
+
+#else /* !USE_CAS */
+
+/*
+ * Acquire lock using load-/store-exclusive instruction pair.
+ *
+ * void spin_lock(spinlock_t *lock);
+ */
 func spin_lock
 	mov	w2, #1
 	sevl
@@ -45,8 +104,17 @@ l2:	ldaxr	w1, [x0]
 	ret
 endfunc spin_lock
 
+#endif /* USE_CAS */
 
+/*
+ * Release lock previously acquired by spin_lock.
+ *
+ * Unconditionally write 0, and conditionally generate an event.
+ *
+ * void spin_unlock(spinlock_t *lock);
+ */
 func spin_unlock
 	stlr	wzr, [x0]
+	COND_SEV()
 	ret
 endfunc spin_unlock