From b5cc70d7532938c675205c291d632fd2bcfef37c Mon Sep 17 00:00:00 2001
From: Julia Vassiliki <julia.vassiliki@unsw.edu.au>
Date: Fri, 13 Feb 2026 10:56:08 +1100
Subject: [PATCH] libsel4: make thread-local ipc buffer optional

At the moment, the seL4 microkit does not setup TLS variable
support. The workaround has been to `#define __thread` (blank)
before including `<sel4/sel4.h>` in `<microkit.h>`, which causes
issues if `<microkit.h>` is included *after* `<sel4/sel4.h>`,
often with obscure linker errors to `__emutls_**` symbols.

Instead we add a libsel4 config option that allows us to build
it with __thread copied out. We introduce an LIBSEL4_THREAD_LOCAL
macro in a similar way to the existing LIBSEL4_INLINE macro.

Signed-off-by: Julia Vassiliki <julia.vassiliki@unsw.edu.au>
---
 libsel4/CMakeLists.txt           | 9 +++++++++
 libsel4/include/sel4/functions.h | 4 ++--
 libsel4/include/sel4/macros.h    | 6 ++++++
 libsel4/src/sel4_bootinfo.c      | 4 ++--
 4 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/libsel4/CMakeLists.txt b/libsel4/CMakeLists.txt
index 56f572058..3ff854a73 100644
--- a/libsel4/CMakeLists.txt
+++ b/libsel4/CMakeLists.txt
@@ -44,6 +44,15 @@ config_string(
     UNQUOTE
 )
 
+config_option(
+    LibSel4UseThreadLocals
+    LIB_SEL4_USE_THREAD_LOCALS
+    "mark the __sel4_ipc_buffer variable as thread local using '__thread'. \
+     This requires thread local storage support and is useful if multiple \
+     threads share the same virtual address space."
+    DEFAULT ON
+)
+
 if(LibSel4StubsUseIPCBufferOnly)
     set(buffer "--buffer")
 endif()
diff --git a/libsel4/include/sel4/functions.h b/libsel4/include/sel4/functions.h
index cd6c14452..f435c37fa 100644
--- a/libsel4/include/sel4/functions.h
+++ b/libsel4/include/sel4/functions.h
@@ -10,10 +10,10 @@
 #include <sel4/types.h>
 #include <sel4/syscalls.h>
 
-extern __thread seL4_IPCBuffer *__sel4_ipc_buffer;
+extern LIBSEL4_THREAD_LOCAL seL4_IPCBuffer *__sel4_ipc_buffer;
 
 #ifdef CONFIG_KERNEL_INVOCATION_REPORT_ERROR_IPC
-extern __thread char __sel4_print_error;
+extern LIBSEL4_THREAD_LOCAL char __sel4_print_error;
 
 LIBSEL4_INLINE_FUNC char *seL4_GetDebugError(void)
 {
diff --git a/libsel4/include/sel4/macros.h b/libsel4/include/sel4/macros.h
index 2ccc281f6..a9f4f3477 100644
--- a/libsel4/include/sel4/macros.h
+++ b/libsel4/include/sel4/macros.h
@@ -42,6 +42,12 @@
 
 #endif
 
+#if defined(CONFIG_LIB_SEL4_USE_THREAD_LOCALS)
+#define LIBSEL4_THREAD_LOCAL    __thread
+#else
+#define LIBSEL4_THREAD_LOCAL
+#endif
+
 /* _Static_assert() is a c11 feature. Since the kernel is currently compiled
  * with c99, we have to emulate it. */
 #if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L)
diff --git a/libsel4/src/sel4_bootinfo.c b/libsel4/src/sel4_bootinfo.c
index afee42729..5cd81d057 100644
--- a/libsel4/src/sel4_bootinfo.c
+++ b/libsel4/src/sel4_bootinfo.c
@@ -7,11 +7,11 @@
 #include <sel4/sel4.h>
 
 #ifdef CONFIG_KERNEL_INVOCATION_REPORT_ERROR_IPC
-__thread char __sel4_print_error = CONFIG_LIB_SEL4_PRINT_INVOCATION_ERRORS;
+LIBSEL4_THREAD_LOCAL char __sel4_print_error = CONFIG_LIB_SEL4_PRINT_INVOCATION_ERRORS;
 #endif
 
 /** Userland per-thread IPC buffer address **/
-__thread seL4_IPCBuffer *__sel4_ipc_buffer;
+LIBSEL4_THREAD_LOCAL seL4_IPCBuffer *__sel4_ipc_buffer;
 
 /** Consider moving bootinfo into libsel4_startup */
 seL4_BootInfo *bootinfo;