implement idle thread in assembly

The idle thread does not have any stack, thus implementing the function
in assembler avoids convincing the compiler not to use any stack for a
C function.

Signed-off-by: Axel Heider <[email protected]>

include/arch/arm/armv/armv7-a/armv/machine.h

@@ -8,7 +8,6 @@
 
 #include <util.h>
 
-/* See idle_thread for an explanation as to why FORCE_INLINE is required here. */
 static inline void FORCE_INLINE wfi(void)
 {
     asm volatile("wfi" ::: "memory");

include/util.h

@@ -85,11 +85,6 @@
 #define SECTION(sec) __attribute__((__section__(sec)))
 #define UNUSED       __attribute__((unused))
 #define USED         __attribute__((used))
-#ifdef __clang__
-#define FORCE_O2     /* nothing */
-#else
-#define FORCE_O2     __attribute__((optimize("O2")))
-#endif
 /** MODIFIES: */
 void __builtin_unreachable(void);
 #define UNREACHABLE()  __builtin_unreachable()

src/arch/arm/32/config.cmake

@@ -16,7 +16,6 @@ add_sources(
         machine/fpu.c
         model/statedata.c
         c_traps.c
-        idle.c
         kernel/thread.c
         kernel/vspace.c
     ASMFILES head.S traps.S hyp_traps.S

src/arch/arm/64/config.cmake

@@ -16,7 +16,6 @@ add_sources(
         machine/fpu.c
         model/statedata.c
         c_traps.c
-        idle.c
         kernel/thread.c
         kernel/vspace.c
     ASMFILES head.S traps.S

src/arch/arm/config.cmake

@@ -265,6 +265,7 @@ add_sources(
         object/smmu.c
         object/smc.c
         smp/ipi.c
+    ASMFILES idle.S
 )
 
 add_bf_source_old("KernelArchARM" "structures.bf" "include/arch/arm" "arch/object")

src/arch/arm/idle.S

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2014, General Dynamics C4 Systems
+ * Copyright 2020, Data61, CSIRO (ABN 41 687 119 230)
+ * Copyright 2021, Axel Heider <[email protected]>
+ *
+ * SPDX-License-Identifier: GPL-2.0-only
+ */
+
+#include <machine/assembler.h>
+
+/* The idle thread does not have any stack, thus its simple loop is implemented
+ * in assembly. That avoids having to find ways to convince the compiler not to
+ * ever generate any code that uses the stack. There seems no clean and portable
+ * way for this, and there is no guarantee any solution in C will always work.
+ */
+.section .text, "ax"
+BEGIN_FUNC(idle_thread)
+1:  wfi
+    b 1b
+END_FUNC(idle_thread)

src/arch/riscv/config.cmake

@@ -95,7 +95,6 @@ add_sources(
     PREFIX src/arch/riscv
     CFILES
         c_traps.c
-        idle.c
         api/faults.c
         api/benchmark.c
         kernel/boot.c
@@ -111,7 +110,7 @@ add_sources(
         object/objecttype.c
         object/tcb.c
         smp/ipi.c
-    ASMFILES head.S traps.S
+    ASMFILES head.S traps.S idle.S
 )
 
 add_bf_source_old("KernelArchRiscV" "structures.bf" "include/arch/riscv" "arch/object")

src/arch/riscv/idle.S

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2015, 2016 Hesham Almatary <[email protected]>
+ * Copyright 2020, Data61, CSIRO (ABN 41 687 119 230)
+ * Copyright 2021, Axel Heider <[email protected]>
+ *
+ * SPDX-License-Identifier: GPL-2.0-only
+ */
+
+#include <machine/assembler.h>
+
+/* The idle thread does not have any stack, thus its simple loop is implemented
+ * in assembly. That avoids having to find ways to convince the compiler not to
+ * ever generate any code that uses the stack. There seems no clean and portable
+ * way for this, and there is no guarantee any solution in C will always work.
+ */
+.section .text, "ax"
+BEGIN_FUNC(idle_thread)
+1:  wfi
+    j 1b
+END_FUNC(idle_thread)

src/arch/x86/config.cmake

@@ -365,7 +365,6 @@ add_sources(
     PREFIX src/arch/x86
     CFILES
         c_traps.c
-        idle.c
         api/faults.c
         object/interrupt.c
         object/ioport.c
@@ -392,7 +391,7 @@ add_sources(
         machine/registerset.c
         benchmark/benchmark.c
         smp/ipi.c
-    ASMFILES multiboot.S
+    ASMFILES multiboot.S idle.S
 )
 
 add_bf_source_old("KernelArchX86" "structures.bf" "include/arch/x86" "arch/object")

src/arch/x86/idle.S

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2020, Data61, CSIRO (ABN 41 687 119 230)
+ * Copyright 2021, Axel Heider <[email protected]>
+ *
+ * SPDX-License-Identifier: GPL-2.0-only
+ */
+
+#include <machine/assembler.h>
+
+/* The idle thread does not have any stack, thus its simple loop is implemented
+ * in assembly. That avoids having to find ways to convince the compiler not to
+ * ever generate any code that uses the stack. There seems no clean and portable
+ * way for this, and there is no guarantee any solution in C will always work.
+ */
+.section .text, "ax"
+BEGIN_FUNC(idle_thread)
+1:  hlt
+    jmp 1b
+END_FUNC(idle_thread)