Add additional tests for TSS entry

Signed-off-by: Joe Richey <[email protected]>

testing/src/gdt.rs

@@ -20,6 +20,8 @@ lazy_static! {
     };
     static ref GDT: (GlobalDescriptorTable, Selectors) = {
         let mut gdt = GlobalDescriptorTable::new();
+        // Add an unused segment so we get a different value for CS
+        gdt.append(Descriptor::kernel_data_segment());
         let code_selector = gdt.append(Descriptor::kernel_code_segment());
         let tss_selector = gdt.append(Descriptor::tss_segment(&TSS));
         (
@@ -41,9 +43,16 @@ pub fn init() {
     use x86_64::instructions::segmentation::{Segment, CS};
     use x86_64::instructions::tables::load_tss;
 
+    // Make sure loading CS actually changes the value
     GDT.0.load();
-    unsafe {
-        CS::set_reg(GDT.1.code_selector);
-        load_tss(GDT.1.tss_selector);
-    }
+    assert_ne!(CS::get_reg(), GDT.1.code_selector);
+    unsafe { CS::set_reg(GDT.1.code_selector) };
+    assert_eq!(CS::get_reg(), GDT.1.code_selector);
+
+    // Loading the TSS should mark the GDT entry as busy
+    let tss_idx: usize = GDT.1.tss_selector.index().into();
+    let old_tss_entry = GDT.0.entries()[tss_idx].clone();
+    unsafe { load_tss(GDT.1.tss_selector) };
+    let new_tss_entry = GDT.0.entries()[tss_idx].clone();
+    assert_ne!(old_tss_entry, new_tss_entry);
 }