GDT enumeration

kernel/src/arch/x86_64/asm.rs

@@ -1,6 +1,20 @@
 // Copyright (c) 2025 shibedrill
 // SPDX-License-Identifier: MIT
 
+use core::arch::asm;
+
+use intbits::Bits;
+
 pub fn nop() {
     ::x86_64::instructions::nop();
 }
+
+pub fn long_mode() -> bool {
+    let mut output: u32;
+    unsafe { asm!(
+        "mov eax, 0x80000001",
+        "cpuid",
+        out("edx") output,
+    ) }
+    output.bit(29)
+}

kernel/src/arch/x86_64/gdt.rs

@@ -5,8 +5,13 @@ use core::arch::asm;
 use core::ptr::addr_of_mut;
 
 use intbits::Bits;
+use x86_64::structures::gdt;
 use x86_64::structures::gdt::*;
 
+use crate::log_info;
+use crate::log::*;
+use crate::memory::alloc::format;
+
 #[allow(dead_code)]
 pub trait GdtEntryRead {
     fn base(&self) -> usize;
@@ -48,3 +53,64 @@ pub fn read_gdt() -> GlobalDescriptorTable {
         unsafe { core::slice::from_raw_parts(base as *const u64, (limit / 8) - 1) };
     GlobalDescriptorTable::from_raw_entries(gdt_raw)
 }
+
+pub fn dump_gdt() {
+    let gdt = crate::arch::x86_64::gdt::read_gdt();
+    log_info!("GDT length: {}", gdt.limit() + 1);
+    for entry in gdt.entries() {
+        let flags = x86_64::structures::gdt::DescriptorFlags::from_bits_retain(entry.raw());
+        let seg_type = SegmentType::try_from(entry);
+        log_info!(
+            "GDT entry: type: {:?}, size: {}, raw flags: 0b{:04b}, access: 0b{:08b}",
+            seg_type.unwrap(),
+            if flags.contains(gdt::DescriptorFlags::DEFAULT_SIZE) {
+                "32-bit"
+            } else if flags.contains(gdt::DescriptorFlags::LONG_MODE) {
+                "64-bit"
+            } else {
+                "16-bit"
+            },
+            entry.raw().bits(52..=55),
+            entry.access()
+        );
+    }
+
+}
+
+#[derive(Debug)]
+enum SegmentType {
+    Null,
+    Code,
+    Data,
+    Ldt,
+    Tss16Avail,
+    Tss16Busy,
+    Tss32Avail,
+    Tss32Busy,
+    Tss64Avail,
+    Tss64Busy,
+}
+
+impl TryFrom<&gdt::Entry> for SegmentType {
+    type Error = ();
+    fn try_from(value: &gdt::Entry) -> Result<Self, ()> {
+        let flags = x86_64::structures::gdt::DescriptorFlags::from_bits_retain(value.raw());
+        if value.access().bit(4) {
+            if value.access().bit(3) {
+                    Ok(SegmentType::Code)
+                } else {
+                    Ok(SegmentType::Data)
+                }
+            } else {
+                match value.access().bits(0..=3) {
+                    0b0000 => Ok(SegmentType::Null),
+                    0b0001 => Ok(SegmentType::Tss16Avail),
+                    0b0010 => Ok(SegmentType::Ldt),
+                    0b0011 => Ok(SegmentType::Tss16Busy),
+                    0b1001 => {if flags.contains(gdt::DescriptorFlags::LONG_MODE) { Ok(SegmentType::Tss64Avail) } else { Ok(SegmentType::Tss32Avail) }},
+                    0b1011 => {if flags.contains(gdt::DescriptorFlags::LONG_MODE) { Ok(SegmentType::Tss64Busy) } else { Ok(SegmentType::Tss32Busy) }},
+                    _ => Err(()),
+                }
+            }
+    }
+}

kernel/src/main.rs

@@ -29,11 +29,9 @@ use limine::firmware_type::FirmwareType;
 use spin::mutex::Mutex;
 
 use crate::{
-    arch::x86_64::{
-        cpuid::{CPUID, virt_supported},
-        gdt::GdtEntryRead,
-    },
-    memory::HHDM_RESPONSE,
+    arch::{asm, x86_64::{
+        cpuid::{CPUID, virt_supported}, gdt::dump_gdt,
+    }}, memory::HHDM_RESPONSE,
 };
 
 lazy_static! {
@@ -165,16 +163,8 @@ unsafe extern "C" fn main() -> ! {
     );
 
     //iter_table(4, &PML4);
-
-    let gdt = crate::arch::x86_64::gdt::read_gdt();
-    log_info!("GDT length: {}", gdt.limit() + 1);
-    for entry in gdt.entries() {
-        log_info!(
-            "GDT entry: Flags 0b{:b}, Access 0b{:b}",
-            entry.flags(),
-            entry.access()
-        );
-    }
+    log_info!("Long mode: {}", asm::long_mode());
+    dump_gdt();
 
     panic!("Finished boot, but cannot start init because processes not implemented!");
 }