save fsbase to TrapFrame on x86_64

kernel/src/arch/x86_64/interrupt/trap.asm

@@ -20,6 +20,14 @@ __alltraps:
     push r14
     push r15
 
+    # push fs.base
+    xor rax, rax
+    mov ecx, 0xC0000100
+    rdmsr # msr[ecx] => edx:eax
+    shl rdx, 32
+    or rdx, rax
+    push rdx
+
     mov rdi, rsp
     call rust_trap
 
@@ -29,6 +37,13 @@ trap_ret:
     mov rdi, rsp
     call set_return_rsp
 
+    # pop fs.base
+    pop rax
+    mov rdx, rax
+    shr rdx, 32
+    mov ecx, 0xC0000100
+    wrmsr # msr[ecx] <= edx:eax
+
     pop r15
     pop r14
     pop r13

kernel/src/arch/x86_64/interrupt/trapframe.rs

@@ -2,6 +2,8 @@
 #[repr(C)]
 pub struct TrapFrame {
     // Pushed by __alltraps at 'trap.asm'
+    pub fsbase: usize,
+
     pub r15: usize,
     pub r14: usize,
     pub r13: usize,