Ring 3 multitasking

src/kernel/syscall.c

@@ -0,0 +1,55 @@
+#include "syscall.h"
+#include "gdt.h"
+#include "memory_manager.h"
+
+// Read MSR
+static inline uint64_t rdmsr(uint32_t msr) {
+    uint32_t low, high;
+    asm volatile("rdmsr" : "=a"(low), "=d"(high) : "c"(msr));
+    return ((uint64_t)high << 32) | low;
+}
+
+// Write MSR
+static inline void wrmsr(uint32_t msr, uint64_t value) {
+    uint32_t low = value & 0xFFFFFFFF;
+    uint32_t high = value >> 32;
+    asm volatile("wrmsr" : : "c"(msr), "a"(low), "d"(high));
+}
+
+// Implemented in assembly
+extern void syscall_entry(void);
+
+extern uint64_t kernel_syscall_stack;
+
+void syscall_init(void) {
+    void* stack = kmalloc(16384);
+    kernel_syscall_stack = (uint64_t)stack + 16384;
+    uint64_t efer = rdmsr(MSR_EFER);
+    efer |= 1; // SCE bit is bit 0
+    wrmsr(MSR_EFER, efer);
+
+    // STAR MSR setup:
+    // Bits 32-47: Syscall CS and SS. CS = STAR[47:32], SS = STAR[47:32] + 8 (Kernel CS = 0x08)
+    // Bits 48-63: Sysret CS and SS. CS = STAR[63:48] + 16, SS = STAR[63:48] + 8.
+    // User Data must be Base+8, User Code must be Base+16.
+    // Our GDT: User Data = 0x1B, User Code = 0x23. 
+    // Therefore Base = 0x13.
+    uint64_t star = ((uint64_t)0x08 << 32) | ((uint64_t)0x13 << 48);
+    wrmsr(MSR_STAR, star);
+
+    wrmsr(MSR_LSTAR, (uint64_t)syscall_entry);
+
+    // Mask Interrupts on SYSCALL (Clear IF bit in RFLAGS during syscall execution)
+    wrmsr(MSR_FMASK, 0x200);
+}
+
+void syscall_handler_c(uint64_t syscall_num, uint64_t arg1, uint64_t arg2, uint64_t arg3, uint64_t arg4, uint64_t arg5) {
+    extern void cmd_write(const char *str);
+    extern void serial_write(const char *str);
+    
+    if (syscall_num == 1) { // SYS_WRITE
+        // arg2 is the buffer based on our user_test logic
+        cmd_write((const char*)arg2);
+        serial_write((const char*)arg2);
+    }
+}