refactor

kernel/arch/x86_64/sys.c

@@ -1,6 +1,7 @@
 #include <zinnia/archctl.h>
 #include <zinnia/status.h>
 #include <common/compiler.h>
+#include <kernel/arch.h>
 #include <kernel/init.h>
 #include <kernel/percpu.h>
 #include <x86_64/apic.h>
@@ -9,7 +10,7 @@
 #include <x86_64/gdt.h>
 
 [[__init, __naked, __used]]
-void _start() {
+void arch_start() {
     asm volatile(
         "lea rsp, [rip + %0]\n"
         "jmp %1"
@@ -25,6 +26,10 @@ zn_status_t arch_archctl(zn_archctl_t op, uintptr_t arg) {
     case ZN_ARCHCTL_SET_FSBASE:
         asm_wrmsr(MSR_FS_BASE, arg);
         return 0;
+    case ZN_ARCHCTL_SET_GSBASE:
+        // Slightly misleading, but KERNEL_GS_BASE is the currently inactive register.
+        asm_wrmsr(MSR_KERNEL_GS_BASE, arg);
+        return 0;
     default:
         return ZN_ERR_INVALID;
     }