From 62cf1960ad0b2c2725024fb3329441b8ae145438 Mon Sep 17 00:00:00 2001
From: Daniel Leung <daniel.leung@intel.com>
Date: Mon, 5 Oct 2020 14:54:45 -0700
Subject: [PATCH] kernel: support using thread local storage for errno

This enables storing errno in the thread local storage area.
With this enabled, a syscall to access errno can be avoided
when userspace is also enabled.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
---
 include/kernel.h            |  4 +++-
 include/sys/errno_private.h | 12 ++++++++++++
 kernel/Kconfig              | 12 ++++++++++--
 kernel/errno.c              |  8 ++++++++
 kernel/thread.c             |  2 ++
 5 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/include/kernel.h b/include/kernel.h
index 9f07e6185a2..381af790724 100644
--- a/include/kernel.h
+++ b/include/kernel.h
@@ -266,7 +266,9 @@ struct _mem_domain_info {
 
 #ifdef CONFIG_THREAD_USERSPACE_LOCAL_DATA
 struct _thread_userspace_local_data {
+#if defined(CONFIG_ERRNO) && !defined(CONFIG_ERRNO_IN_TLS)
 	int errno_var;
+#endif
 };
 #endif
 
@@ -329,7 +331,7 @@ struct k_thread {
 	struct _thread_userspace_local_data *userspace_local_data;
 #endif
 
-#ifdef CONFIG_ERRNO
+#if defined(CONFIG_ERRNO) && !defined(CONFIG_ERRNO_IN_TLS)
 #ifndef CONFIG_USERSPACE
 	/** per-thread errno variable */
 	int errno_var;
diff --git a/include/sys/errno_private.h b/include/sys/errno_private.h
index a51f37765b1..f1c1bff4cb0 100644
--- a/include/sys/errno_private.h
+++ b/include/sys/errno_private.h
@@ -17,6 +17,14 @@ extern "C" {
  * and kernel.h
  */
 
+#ifdef CONFIG_ERRNO_IN_TLS
+extern __thread int z_errno_var;
+
+static inline int *z_errno(void)
+{
+	return &z_errno_var;
+}
+#else
 /**
  * return a pointer to a memory location containing errno
  *
@@ -27,10 +35,14 @@ extern "C" {
  */
 __syscall int *z_errno(void);
 
+#endif /* CONFIG_ERRNO_IN_TLS */
+
 #ifdef __cplusplus
 }
 #endif
 
+#ifndef CONFIG_ERRNO_IN_TLS
 #include <syscalls/errno_private.h>
+#endif /* CONFIG_ERRNO_IN_TLS */
 
 #endif /* ZEPHYR_INCLUDE_SYS_ERRNO_PRIVATE_H_ */
diff --git a/kernel/Kconfig b/kernel/Kconfig
index 18441ada35f..db845a6bdef 100644
--- a/kernel/Kconfig
+++ b/kernel/Kconfig
@@ -195,16 +195,24 @@ config THREAD_CUSTOM_DATA
 config THREAD_USERSPACE_LOCAL_DATA
 	bool
 	depends on USERSPACE
+	default y if ERRNO && !ERRNO_IN_TLS
 
 config ERRNO
 	bool "Enable errno support"
 	default y
-	select THREAD_USERSPACE_LOCAL_DATA if USERSPACE
 	help
 	  Enable per-thread errno in the kernel. Application and library code must
 	  include errno.h provided by the C library (libc) to use the errno
 	  symbol. The C library must access the per-thread errno via the
-	  _get_errno() symbol.
+	  z_errno() symbol.
+
+config ERRNO_IN_TLS
+	bool "Store errno in thread local storage (TLS)"
+	depends on ERRNO && THREAD_LOCAL_STORAGE
+	default y
+	help
+	  Use thread local storage to store errno instead of storing it in
+	  the kernel thread struct. This avoids a syscall if userspace is enabled.
 
 choice SCHED_ALGORITHM
 	prompt "Scheduler priority queue algorithm"
diff --git a/kernel/errno.c b/kernel/errno.c
index 51a2363d28d..84f78593dc1 100644
--- a/kernel/errno.c
+++ b/kernel/errno.c
@@ -23,6 +23,11 @@
 const int _k_neg_eagain = -EAGAIN;
 
 #ifdef CONFIG_ERRNO
+
+#ifdef CONFIG_ERRNO_IN_TLS
+__thread int z_errno_var;
+#else
+
 #ifdef CONFIG_USERSPACE
 int *z_impl_z_errno(void)
 {
@@ -44,4 +49,7 @@ int *z_impl_z_errno(void)
 	return &_current->errno_var;
 }
 #endif /* CONFIG_USERSPACE */
+
+#endif /* CONFIG_ERRNO_IN_TLS */
+
 #endif /* CONFIG_ERRNO */
diff --git a/kernel/thread.c b/kernel/thread.c
index 16de7a8f93e..6e3b7e23b7d 100644
--- a/kernel/thread.c
+++ b/kernel/thread.c
@@ -835,8 +835,10 @@ FUNC_NORETURN void k_thread_user_mode_enter(k_thread_entry_t entry,
 #ifdef CONFIG_USERSPACE
 	__ASSERT(z_stack_is_user_capable(_current->stack_obj),
 		 "dropping to user mode with kernel-only stack object");
+#ifdef CONFIG_THREAD_USERSPACE_LOCAL_DATA
 	memset(_current->userspace_local_data, 0,
 	       sizeof(struct _thread_userspace_local_data));
+#endif
 	arch_user_mode_enter(entry, p1, p2, p3);
 #else
 	/* XXX In this case we do not reset the stack */