arch: riscv: implement ARCH_HAS_CUSTOM_CURRENT_IMPL with GP

Implement `arch_curr_thread()` & `arch_set_curr_thread()`
with the global pointer (GP) register.

Signed-off-by: Yong Cong Sin <[email protected]>
Signed-off-by: Yong Cong Sin <[email protected]>

Author: ycsin

arch/riscv/Kconfig

@@ -28,6 +28,16 @@ config RISCV_GP
 	  global pointer at program start or earlier than any instruction
 	  using GP relative addressing.
 
+config RISCV_CURRENT_VIA_GP
+	bool "Store current thread into the global pointer (GP) register"
+	depends on !RISCV_GP && !USERSPACE
+	depends on MP_MAX_NUM_CPUS > 1
+	select ARCH_HAS_CUSTOM_CURRENT_IMPL
+	help
+	  Store the current thread's pointer into the global pointer (GP) register.
+	  When is enabled, calls to `_current` & `k_sched_current_thread_query()` will
+	  be reduced to a single register read.
+
 config RISCV_ALWAYS_SWITCH_THROUGH_ECALL
 	bool "Do not use mret outside a trap handler context"
 	depends on MULTITHREADING

doc/releases/release-notes-4.1.rst

@@ -52,6 +52,9 @@ Architectures
 
 * RISC-V
 
+  * Implements :c:func:`arch_current_thread_set` & :c:func:`arch_current_thread`, which can be enabled
+    by :kconfig:option:`CONFIG_RISCV_CURRENT_VIA_GP` (:github:`80716`).
+
 * Xtensa
 
 * native/POSIX

include/zephyr/arch/riscv/arch_inlines.h

@@ -11,6 +11,7 @@
 
 #include <zephyr/kernel_structs.h>
 #include "csr.h"
+#include "reg.h"
 
 static ALWAYS_INLINE uint32_t arch_proc_id(void)
 {
@@ -26,6 +27,17 @@ static ALWAYS_INLINE _cpu_t *arch_curr_cpu(void)
 #endif
 }
 
+#ifdef CONFIG_RISCV_CURRENT_VIA_GP
+register struct k_thread *__arch_current_thread __asm__("gp");
+
+#define arch_current_thread() __arch_current_thread
+#define arch_current_thread_set(thread)                                                            \
+	{                                                                                          \
+		_current_cpu->current = thread;                                                    \
+		__arch_current_thread = (thread);                                                  \
+	}
+#endif /* CONFIG_RISCV_CURRENT_VIA_GP */
+
 static ALWAYS_INLINE unsigned int arch_num_cpus(void)
 {
 	return CONFIG_MP_MAX_NUM_CPUS;