Temporarily fix sysretq by swapping gs 4 times.

In order words, it swaps gs both directly at the start of the syscall
handler, then swaps it back, and the at the end of the syscall handler.
I cannot tell for sure why this is necessary, but probably since some
interrupt handler will execute swapgs in the wrong order or something.

src/arch/x86_64/gdt.rs

@@ -194,7 +194,7 @@ pub unsafe fn init_paging(tcb_offset: usize, stack_offset: usize) {
 
     segmentation::load_gs(SegmentSelector::new(GDT_KERNEL_DATA as u16, Ring::Ring0));
     // Ensure that GS always points to the TSS segment in kernel space.
-    x86::msr::wrmsr(x86::msr::IA32_GS_BASE, &TSS as *const _ as usize as u64);
+    //x86::msr::wrmsr(x86::msr::IA32_GS_BASE, &TSS as *const _ as usize as u64);
 
     segmentation::load_ss(SegmentSelector::new(GDT_KERNEL_DATA as u16, Ring::Ring0));
 

src/arch/x86_64/interrupt/exception.rs

@@ -41,8 +41,6 @@ interrupt_stack!(debug, |stack| {
     }
 });
 
-// TODO: Give NMI and double faults a separate stack, as they can trigger a triple fault due to the
-// way swapgs is used.
 interrupt_stack!(non_maskable, |stack| {
     println!("Non-maskable interrupt");
     stack.dump();

src/arch/x86_64/interrupt/handler.rs

@@ -309,14 +309,38 @@ macro_rules! pop_fs {
     " };
 }
 
-macro_rules! swapgs_if_ring3 {
-    () => { "
+macro_rules! swapgs_iff_ring3 {
+    (error_code: true) => { "
         // Check whether the last two bits RSP+8 (code segment) are equal to zero.
-        test BYTE PTR [rsp + 8], 0x03
+        test QWORD PTR [rsp + 8], 0x3
         // Skip the SWAPGS instruction if CS & 0b11 == 0b00.
         jz 1f
         swapgs
         1:
+    " };
+    (error_code: false) => { "
+        test QWORD PTR [rsp + 16], 0x3
+        jz 1f
+        swapgs
+        1:
+    " };
+}
+macro_rules! swapgs_iff_ring3_slow {
+    () => { "
+    push eax
+    push edx
+    push ecx
+    mov ecx, 0xC000_0102
+    rdmsr
+    shl rdx, 32
+    or eax, edx
+    test rdx, rdx
+    jnz 1f
+    swapgs
+    1:
+    pop ecx
+    pop edx
+    pop eax
     " }
 }
 
@@ -338,7 +362,7 @@ macro_rules! interrupt_stack {
 
             function!($name => {
                 // Backup all userspace registers to stack
-                swapgs_if_ring3!(),
+                //swapgs_if_ring3!(error_code: false),
                 "push rax\n",
                 push_scratch!(),
                 push_preserved!(),
@@ -359,7 +383,7 @@ macro_rules! interrupt_stack {
                 pop_preserved!(),
                 pop_scratch!(),
 
-                swapgs_if_ring3!(),
+                //swapgs_if_ring3!(error_code: false),
                 "iretq\n",
             });
         }
@@ -377,7 +401,7 @@ macro_rules! interrupt {
 
             function!($name => {
                 // Backup all userspace registers to stack
-                swapgs_if_ring3!(),
+                //swapgs_if_ring3!(error_code: false),
                 "push rax\n",
                 push_scratch!(),
                 push_fs!(),
@@ -395,7 +419,7 @@ macro_rules! interrupt {
                 pop_fs!(),
                 pop_scratch!(),
 
-                swapgs_if_ring3!(),
+                //swapgs_if_ring3!(error_code: false),
                 "iretq\n",
             });
         }
@@ -419,7 +443,7 @@ macro_rules! interrupt_error {
             }
 
             function!($name => {
-                swapgs_if_ring3!(),
+                //swapgs_if_ring3!(error_code: true),
                 // Move rax into code's place, put code in last instead (to be
                 // compatible with InterruptStack)
                 "xchg [rsp], rax\n",
@@ -450,7 +474,7 @@ macro_rules! interrupt_error {
                 pop_preserved!(),
                 pop_scratch!(),
 
-                swapgs_if_ring3!(),
+                //swapgs_if_ring3!(error_code: true),
                 "iretq\n",
             });
         }

src/arch/x86_64/interrupt/syscall.rs

@@ -25,7 +25,7 @@ pub unsafe fn init() {
 
     // Inside kernel space, GS should _always_ point to the TSS. When leaving userspace, `swapgs`
     // is called again, making the userspace GS always point to user data.
-    msr::wrmsr(msr::IA32_KERNEL_GSBASE, 0);
+    x86::msr::wrmsr(x86::msr::IA32_KERNEL_GSBASE, &gdt::TSS as *const _ as usize as u64);
 
     let efer = msr::rdmsr(msr::IA32_EFER);
     msr::wrmsr(msr::IA32_EFER, efer | 1);
@@ -68,11 +68,12 @@ function!(syscall_instruction => {
         swapgs                    // Set gs segment to TSS
         mov gs:[0x70], rsp        // Save userspace stack pointer
         mov rsp, gs:[4]           // Load kernel stack pointer
-        push WORD PTR 5 * 8 + 3   // Push fake userspace SS (resembling iret frame)
+        push QWORD PTR 5 * 8 + 3  // Push fake userspace SS (resembling iret frame)
         push QWORD PTR gs:[0x70]  // Push userspace rsp
         push r11                  // Push rflags
-        push WORD PTR 6 * 8 + 3   // Push fake userspace CS (resembling iret frame)
+        push QWORD PTR 6 * 8 + 3  // Push fake userspace CS (resembling iret frame)
         push rcx                  // Push userspace return pointer
+        swapgs
     ",
 
     // Push context registers
@@ -99,10 +100,11 @@ function!(syscall_instruction => {
     // Return
     "
         pop rcx                 // Pop userspace return pointer
-        add rsp, 2              // Pop CS
+        add rsp, 8              // Pop CS
         pop r11                 // Pop rflags
+        swapgs
         pop QWORD PTR gs:[0x70] // Pop userspace stack pointer
-        add rsp, 2              // Pop SS
+        add rsp, 8              // Pop SS
         mov rsp, gs:[0x70]      // Restore userspace stack pointer
         swapgs                  // Restore gs from TSS to user data
         sysretq                 // Return into userspace; RCX=>RIP,R11=>RFLAGS

src/arch/x86_64/start.rs

@@ -267,9 +267,6 @@ pub unsafe extern "C" fn usermode(_ip: usize, _sp: usize, _arg: usize, _singlest
             mov ds, r8d
             mov es, r8d
             mov fs, r9d
-            // Exchange the old kernel GS (pointing to TSS) and kernel data
-            swapgs
-            // Replace kernel data segment with user data segment
             mov gs, r8d
 
             // Target RFLAGS
@@ -279,7 +276,7 @@ pub unsafe extern "C" fn usermode(_ip: usize, _sp: usize, _arg: usize, _singlest
             // Target stack pointer
             mov rsp, r14
             // Target argument
-            mov rdi, r13
+            mov rdi, r15
 
             xor rax, rax
             xor rbx, rbx