aarch64: userspace: Introduce skeleton code for user-threads

Introduce the first pieces needed to schedule user threads by defining
two different code paths for kernel and user threads.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>

arch/arm/core/aarch64/CMakeLists.txt

@@ -20,6 +20,7 @@ zephyr_library_sources(
   vector_table.S
 )
 
+zephyr_library_sources_ifdef(CONFIG_USERSPACE userspace.S)
 zephyr_library_sources_ifdef(CONFIG_GEN_SW_ISR_TABLE isr_wrapper.S)
 zephyr_library_sources_ifdef(CONFIG_IRQ_OFFLOAD irq_offload.c)
 zephyr_library_sources_ifdef(CONFIG_THREAD_LOCAL_STORAGE ../common/tls.c)

arch/arm/core/aarch64/thread.c

@@ -16,11 +16,13 @@
 #include <wait_q.h>
 #include <arch/cpu.h>
 
-/*
+#ifdef CONFIG_USERSPACE
- * An initial context, to be "restored" by z_arm64_context_switch(), is put at
+static bool is_user(struct k_thread *thread)
- * the other end of the stack, and thus reusable by the stack when not needed
+{
- * anymore.
+	return (thread->base.user_options & K_USER) != 0;
- */
+}
+#endif
+
 void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,
 		     char *stack_ptr, k_thread_entry_t entry,
 		     void *p1, void *p2, void *p3)
@@ -40,7 +42,19 @@ void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,
 	 *   parameters already in place in x1(arg1), x2(arg2), x3(arg3).
 	 * - SPSR_ELn: to enable IRQs (we are masking FIQs).
 	 */
+#ifdef CONFIG_USERSPACE
+	/*
+	 * If the new thread is a user thread we jump into
+	 * arch_user_mode_enter() when still in EL1.
+	 */
+	if (is_user(thread)) {
+		pInitCtx->elr = (uint64_t)arch_user_mode_enter;
+	} else {
+		pInitCtx->elr = (uint64_t)z_thread_entry;
+	}
+#else
 	pInitCtx->elr = (uint64_t)z_thread_entry;
+#endif
 	pInitCtx->spsr = SPSR_MODE_EL1T | DAIF_FIQ_BIT;
 
 	/*
@@ -58,3 +72,37 @@ void *z_arch_get_next_switch_handle(struct k_thread **old_thread)
 
 	return z_get_next_switch_handle(*old_thread);
 }
+
+#ifdef CONFIG_USERSPACE
+FUNC_NORETURN void arch_user_mode_enter(k_thread_entry_t user_entry,
+					void *p1, void *p2, void *p3)
+{
+	z_arch_esf_t *pInitCtx;
+	uintptr_t stack_ptr;
+
+	/* Reset the stack pointer to the base discarding any old context */
+	stack_ptr = Z_STACK_PTR_ALIGN(_current->stack_info.start +
+				      _current->stack_info.size -
+				      _current->stack_info.delta);
+
+	/*
+	 * Reconstruct the ESF from scratch to leverage the z_arm64_exit_exc()
+	 * macro that will simulate a return from exception to move from EL1t
+	 * to EL0t. On return we will be in userspace.
+	 */
+	pInitCtx = Z_STACK_PTR_TO_FRAME(struct __esf, stack_ptr);
+
+	pInitCtx->spsr = DAIF_FIQ_BIT | SPSR_MODE_EL0T;
+	pInitCtx->elr = (uint64_t)z_thread_entry;
+
+	pInitCtx->x0 = (uint64_t)user_entry;
+	pInitCtx->x1 = (uint64_t)p1;
+	pInitCtx->x2 = (uint64_t)p2;
+	pInitCtx->x3 = (uint64_t)p3;
+
+	/* All the needed information is already in the ESF */
+	z_arm64_userspace_enter(pInitCtx);
+
+	CODE_UNREACHABLE;
+}
+#endif

arch/arm/core/aarch64/userspace.S

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2020 Carlo Caione <ccaione@baylibre.com>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <toolchain.h>
+#include <linker/sections.h>
+#include <offsets_short.h>
+#include <arch/cpu.h>
+#include <syscall.h>
+#include "macro_priv.inc"
+
+_ASM_FILE_PROLOGUE
+
+/*
+ * Routine to jump into userspace
+ *
+ * We leverage z_arm64_exit_exc() to pop out the entry function and parameters
+ * from ESF and fake a return from exception to move from EL1 to EL0. The fake
+ * ESF is built in arch_user_mode_enter() before jumping here
+ */
+
+GTEXT(z_arm64_userspace_enter)
+SECTION_FUNC(TEXT, z_arm64_userspace_enter)
+	mov	sp, x0
+	b	z_arm64_exit_exc

arch/arm/include/aarch64/kernel_arch_func.h

@@ -40,6 +40,7 @@ static inline void arch_switch(void *switch_to, void **switched_from)
 }
 
 extern void z_arm64_fatal_error(z_arch_esf_t *esf, unsigned int reason);
+extern void z_arm64_userspace_enter(z_arch_esf_t *esf);
 
 #endif /* _ASMLANGUAGE */