gdt: Remove the segment selector for GS

It turns out that we don't need to have a segment selector for GS
to be able to use the GSBase MSR, as discussed in rust-osdev/x86_64#333.
I suspect this is because we now have a kernel data segment selector
for SS, which we didn't when I first added support for CPU-local
data. Removing the extra selector frees up a slot in the GDT.

Signed-off-by: SlyMarbo <[email protected]>

Author: SlyMarbo

kernel/src/gdt.rs

@@ -19,7 +19,7 @@
 //! have no effect.
 
 use lazy_static::lazy_static;
-use x86_64::instructions::segmentation::{Segment, CS, GS, SS};
+use x86_64::instructions::segmentation::{Segment, CS, SS};
 use x86_64::instructions::tables::load_tss;
 use x86_64::structures::gdt::{Descriptor, GlobalDescriptorTable, SegmentSelector};
 use x86_64::structures::tss::TaskStateSegment;
@@ -33,7 +33,6 @@ pub fn init() {
     unsafe {
         CS::set_reg(GDT.1.kernel_code_selector);
         SS::set_reg(GDT.1.kernel_stack_selector);
-        GS::set_reg(GDT.1.cpu_local_selector);
         load_tss(GDT.1.tss_selector);
     }
 }
@@ -44,14 +43,12 @@ lazy_static! {
         let kernel_code_selector = gdt.add_entry(Descriptor::kernel_code_segment());
         let kernel_stack_selector = gdt.add_entry(Descriptor::kernel_data_segment());
         let tss_selector = gdt.add_entry(Descriptor::tss_segment(&TSS));
-        let cpu_local_selector = gdt.add_entry(Descriptor::kernel_data_segment());
         (
             gdt,
             Selectors {
                 kernel_code_selector,
                 kernel_stack_selector,
                 tss_selector,
-                cpu_local_selector,
             },
         )
     };
@@ -61,7 +58,6 @@ struct Selectors {
     kernel_code_selector: SegmentSelector,
     kernel_stack_selector: SegmentSelector,
     tss_selector: SegmentSelector,
-    cpu_local_selector: SegmentSelector,
 }
 
 // Set up the task state segment with a safe

kernel/src/multitasking/cpu_local.rs

@@ -32,7 +32,7 @@ use core::mem::size_of;
 use core::sync::atomic::{AtomicBool, AtomicU64, Ordering};
 use x86_64::addr::align_up;
 use x86_64::instructions::interrupts;
-use x86_64::instructions::segmentation::{Segment, CS, GS, SS};
+use x86_64::instructions::segmentation::{Segment, CS, SS};
 use x86_64::instructions::tables::load_tss;
 use x86_64::registers::model_specific::GsBase;
 use x86_64::registers::model_specific::Msr;
@@ -161,7 +161,6 @@ pub fn init(cpu_id: CpuId, stack_space: &VirtAddrRange) {
         let kernel_code_selector = data.gdt.add_entry(Descriptor::kernel_code_segment());
         let kernel_stack_selector = data.gdt.add_entry(Descriptor::kernel_data_segment());
         let tss_selector = data.gdt.add_entry(Descriptor::tss_segment(tss_ref));
-        let cpu_local_selector = data.gdt.add_entry(Descriptor::kernel_data_segment());
         let user_code_selector = data.gdt.add_entry(Descriptor::user_code_segment());
         let user_stack_selector = data.gdt.add_entry(Descriptor::user_data_segment());
 
@@ -170,7 +169,6 @@ pub fn init(cpu_id: CpuId, stack_space: &VirtAddrRange) {
         unsafe {
             CS::set_reg(kernel_code_selector);
             SS::set_reg(kernel_stack_selector);
-            GS::set_reg(cpu_local_selector);
             GsBase::write(start); // Set the GS base again now we've updated GS.
             load_tss(tss_selector);