[Kernel] Add idle thread

src/core/kernel/idle_thread.cpp

@@ -0,0 +1,69 @@
+#include <cstring>
+#include "arm_defs.hpp"
+#include "kernel.hpp"
+
+/*
+	This file sets up an idle thread that's meant to run when no other OS thread can run.
+	It simply idles and constantly yields to check if there's any other thread that can run
+	The code for our idle thread looks like this
+
+idle_thread_main:
+	mov r0, #4096            @ Loop counter
+	
+	.loop:
+	nop; nop; nop; nop       @ NOP 4 times to waste some cycles
+	subs r0, #1              @ Decrement counter by 1, go back to looping if loop counter != 0
+	bne .loop
+
+	// Sleep for 0 seconds with the SleepThread SVC, which just yields execution
+	mov r0, #0
+	mov r1, #0
+	svc SleepThread
+
+	b idle_thread_main
+*/
+
+static constexpr u8 idleThreadCode[] = {
+	0x01, 0x0A, 0xA0, 0xE3, // mov r0, #4096
+	0x00, 0xF0, 0x20, 0xE3, 0x00, 0xF0, 0x20, 0xE3, 0x00, 0xF0, 0x20, 0xE3, 0x00, 0xF0, 0x20, 0xE3, // nop (4 times)
+	0x01, 0x00, 0x50, 0xE2,  // subs r0, #1
+	0xF9, 0xFF, 0xFF, 0x1A,  // bne loop
+	0x00, 0x00, 0xA0, 0xE3,  // mov r0, #0
+	0x00, 0x10, 0xA0, 0xE3,  // mov r1, #0
+	0x0A, 0x00, 0x00, 0xEF,  // svc SleepThread
+	0xF4, 0xFF, 0xFF, 0xEA   // b idle_thread_main
+};
+
+// Set up an idle thread to run when no thread is able to run
+void Kernel::setupIdleThread() {
+	Thread& t = threads[idleThreadIndex];
+	constexpr u32 codeAddress = 0xBFC00000;
+
+	// Reserve some memory for the idle thread's code. We map this memory to vaddr BFC00000 which is not userland-accessible
+	// We only allocate 4KB (1 page) because our idle code is pretty small
+	const u32 fcramIndex = mem.allocateSysMemory(Memory::pageSize);
+	auto vaddr = mem.allocateMemory(codeAddress, fcramIndex, Memory::pageSize, true, true, false, true, false, true);
+	if (!vaddr.has_value() || vaddr.value() != codeAddress) {
+		Helpers::panic("Failed to setup idle thread");
+	}
+	
+	// Copy idle thread code to the allocated FCRAM
+	std::memcpy(&mem.getFCRAM()[fcramIndex], idleThreadCode, sizeof(idleThreadCode));
+
+	t.entrypoint = codeAddress;
+	t.gprs[13] = 0; // Set SP & LR to 0 just in case. The idle thread should never access memory, but let's be safe
+	t.gprs[14] = 0;
+	t.gprs[15] = codeAddress;
+	t.cpsr = CPSR::UserMode;
+	t.fpscr = FPSCR::ThreadDefault;
+
+	// Our idle thread should have as low of a priority as possible, because, well, it's an idle thread.
+	// We handle this by giving it a priority of 0xff, which is lower than is actually allowed for user threads
+	// (High priority value = low priority)
+	t.priority = 0xff;
+	t.status = ThreadStatus::Ready;
+
+	// Add idle thread to the list of thread indices
+	threadIndices.push_back(idleThreadIndex);
+	sortThreads();
+}

src/core/kernel/kernel.cpp

@@ -131,6 +131,7 @@ void Kernel::reset() {
 	// which is thankfully not used. Maybe we should prevent this
 	mainThread = makeThread(0, VirtualAddrs::StackTop, 0x30, -2, 0, ThreadStatus::Running);
 	currentThreadIndex = 0;
+	setupIdleThread();
 
 	// Create some of the OS ports
 	srvHandle = makePort("srv:"); // Service manager port