From 8a410a3471fe0e2d485be84d291e4bf827163eac Mon Sep 17 00:00:00 2001
From: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Date: Thu, 6 Aug 2026 19:24:49 +0300
Subject: [PATCH] zephyr: cpu: turn cpu_get_id() into a system call

cpu_get_id() ultimately reads a privileged special register via
arch_proc_id() (e.g. the Xtensa PRID register). When low-atency
pipelines run in user-space threads (CONFIG_SOF_USERSPACE_LL), this read
is issued from user mode and faults. While many direct usages of
cpu_get_id() have been removed from SOF codebase, multiple usages
remain. As the remaining usages are mostly on less frequently used code
paths (most via cpu_is_me() call, which is used in IPC handling), opt to
keep the remaining cpu_get_id() calls and make the function available as
a system call.

The system call machinery is gated on CONFIG_SOF_FULL_ZEPHYR_APPLICATION
so unit-test builds keep the plain inline definition.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
---
 zephyr/CMakeLists.txt        |  2 ++
 zephyr/include/sof/lib/cpu.h | 27 +++++++++++++++++++++++++++
 zephyr/syscall/cpu.c         | 22 ++++++++++++++++++++++
 3 files changed, 51 insertions(+)
 create mode 100644 zephyr/syscall/cpu.c

diff --git a/zephyr/CMakeLists.txt b/zephyr/CMakeLists.txt
index 0ed4a13ee742..e0e7e8bfb302 100644
--- a/zephyr/CMakeLists.txt
+++ b/zephyr/CMakeLists.txt
@@ -620,6 +620,8 @@ zephyr_library_sources_ifdef(CONFIG_SHELL
        sof_shell.c
 )
 
+zephyr_syscall_header(include/sof/lib/cpu.h)
+zephyr_library_sources_ifdef(CONFIG_USERSPACE syscall/cpu.c)
 zephyr_syscall_header(${SOF_SRC_PATH}/include/sof/audio/module_adapter/module/generic.h)
 zephyr_syscall_header(${SOF_SRC_PATH}/include/sof/lib/fast-get.h)
 zephyr_syscall_header(${SOF_SRC_PATH}/include/sof/ipc/ipc_reply.h)
diff --git a/zephyr/include/sof/lib/cpu.h b/zephyr/include/sof/lib/cpu.h
index c23405e85121..a47b93898973 100644
--- a/zephyr/include/sof/lib/cpu.h
+++ b/zephyr/include/sof/lib/cpu.h
@@ -40,13 +40,32 @@ void cpu_notify_state_exit(enum pm_state state);
 
 #endif /* CONFIG_PM */
 
+/*
+ * cpu_get_id() is exposed as a Zephyr system call so that user-mode
+ * threads (e.g. user-space LL pipelines) can query the current core
+ * id. The underlying arch_proc_id() reads a privileged special
+ * register (e.g. Xtensa PRID) which would fault if executed directly
+ * from user mode. In supervisor context the generated wrapper inlines
+ * the z_impl_cpu_get_id() body, so there is no overhead there.
+ */
+#if defined(CONFIG_SOF_FULL_ZEPHYR_APPLICATION)
+__syscall int cpu_get_id(void);
+#endif
+
 /* let the compiler optimise when in single core mode */
 #if CONFIG_MULTICORE && CONFIG_SMP
 
+#if defined(CONFIG_SOF_FULL_ZEPHYR_APPLICATION)
+static inline int z_impl_cpu_get_id(void)
+{
+	return arch_proc_id();
+}
+#else
 static inline int cpu_get_id(void)
 {
 	return arch_proc_id();
 }
+#endif
 
 static inline bool cpu_is_primary(int id)
 {
@@ -73,7 +92,11 @@ int cpu_restore_secondary_cores(void);
 int cpu_secondary_cores_prepare_d0ix(void);
 #else
 
+#if defined(CONFIG_SOF_FULL_ZEPHYR_APPLICATION)
+static inline int z_impl_cpu_get_id(void) { return 0; };
+#else
 static inline int cpu_get_id(void) { return 0; };
+#endif
 
 static inline bool cpu_is_primary(int id) { return 1; };
 
@@ -93,6 +116,10 @@ static inline int cpu_secondary_cores_prepare_d0ix(void) { return 0; };
 
 #endif /* CONFIG_MULTICORE && CONFIG_SMP */
 
+#if defined(CONFIG_SOF_FULL_ZEPHYR_APPLICATION)
+#include <zephyr/syscalls/cpu.h>
+#endif
+
 #endif
 
 #endif /* __SOF_LIB_CPU_H__ */
diff --git a/zephyr/syscall/cpu.c b/zephyr/syscall/cpu.c
new file mode 100644
index 000000000000..ee1990db8fd4
--- /dev/null
+++ b/zephyr/syscall/cpu.c
@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: BSD-3-Clause
+//
+// Copyright(c) 2026 Intel Corporation.
+
+#include <sof/lib/cpu.h>
+#include <zephyr/kernel.h>
+#include <zephyr/internal/syscall_handler.h>
+
+/**
+ * \brief Userspace verification wrapper for cpu_get_id().
+ *
+ * The system call takes no arguments and passes no pointers, so no
+ * access validation is required; the call is simply forwarded to the
+ * implementation running in supervisor context.
+ *
+ * @return Id of the DSP core executing the call.
+ */
+static inline int z_vrfy_cpu_get_id(void)
+{
+	return z_impl_cpu_get_id();
+}
+#include <zephyr/syscalls/cpu_get_id_mrsh.c>