Merge pull request #110 from rust-osdev/rewrite-gdt

Create 32-bit GDT and TSS types

Author: katietheqt-old

Cargo.lock

@@ -5,3 +5,5 @@ authors = ["Ryland Morgan <ryland.hugo@gmail.com>"]
 edition = "2018"
 
 [dependencies]
+bitflags = "1.2.1"
+bit_field = "0.10.0"

src/v86/Cargo.toml

@@ -0,0 +1,230 @@
+use bit_field::BitField;
+use bitflags::bitflags;
+
+#[derive(Debug, Clone)]
+pub struct GlobalDescriptorTable {
+    table: [u64; 8],
+    next_free: usize,
+}
+
+impl GlobalDescriptorTable {
+    /// Creates an empty GDT.
+    #[inline]
+    pub const fn new() -> GlobalDescriptorTable {
+        GlobalDescriptorTable {
+            table: [0; 8],
+            next_free: 1,
+        }
+    }
+
+    /// Adds the given segment descriptor to the GDT, returning the segment selector.
+    ///
+    /// Panics if the GDT has no free entries left.
+    #[inline]
+    pub fn add_entry(&mut self, entry: Descriptor) -> u16 {
+        let index = self.push(entry.0);
+        index as u16
+    }
+
+    /// Loads the GDT in the CPU using the `lgdt` instruction. This does **not** alter any of the
+    /// segment registers; you **must** (re)load them yourself.
+    #[inline]
+    pub fn load(&'static self) {
+        use core::mem::size_of;
+
+        /// A struct describing a pointer to a descriptor table (GDT / IDT).
+        /// This is in a format suitable for giving to 'lgdt' or 'lidt'.
+        #[derive(Debug, Clone, Copy)]
+        #[repr(C, packed)]
+        struct DescriptorTablePointer {
+            /// Size of the DT.
+            pub limit: u16,
+            /// Pointer to the memory region containing the DT.
+            pub base: u32,
+        }
+
+        let ptr = DescriptorTablePointer {
+            base: self.table.as_ptr() as u32,
+            limit: (self.table.len() * size_of::<u64>() - 1) as u16,
+        };
+
+        llvm_asm!("lgdt ($0)" :: "r" (&ptr) : "memory");
+    }
+
+    #[inline]
+    fn push(&mut self, value: u64) -> usize {
+        if self.next_free < self.table.len() {
+            let index = self.next_free;
+            self.table[index] = value;
+            self.next_free += 1;
+            index
+        } else {
+            panic!("GDT full");
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct Descriptor(u64);
+
+bitflags! {
+    /// Flags for a GDT descriptor. Not all flags are valid for all descriptor types.
+    pub struct DescriptorFlags: u64 {
+        /// For data segments, this flag sets the segment as writable. For code
+        /// segments, it defines whether the segment is readable.
+        const READABLE_WRITABLE = 1 << 41;
+        /// Marks a code segment as “conforming”. This influences the privilege checks that
+        /// occur on control transfers.
+        const CONFORMING        = 1 << 42;
+        /// This flag must be set for code segments.
+        const EXECUTABLE        = 1 << 43;
+        /// This flag must be set for user segments (in contrast to system segments).
+        const USER_SEGMENT      = 1 << 44;
+        /// Must be set for any segment, causes a segment not present exception if not set.
+        const PRESENT           = 1 << 47;
+        /// Must be set for long mode code segments.
+        const LONG_MODE         = 1 << 53;
+
+        /// The DPL for this descriptor is Ring 3
+        const DPL_RING_3        = 3 << 45;
+    }
+}
+
+impl Descriptor {
+    /// Creates a segment descriptor for a protected mode kernel code segment.
+    #[inline]
+    pub fn kernel_code_segment() -> Descriptor {
+        use self::DescriptorFlags as Flags;
+
+        let flags =
+            Flags::USER_SEGMENT | Flags::PRESENT | Flags::EXECUTABLE | Flags::READABLE_WRITABLE;
+        Descriptor(flags.bits())
+    }
+
+    /// Creates a segment descriptor for a protected mode kernel data segment.
+    #[inline]
+    pub fn data_segment() -> Descriptor {
+        use self::DescriptorFlags as Flags;
+
+        let flags = Flags::USER_SEGMENT | Flags::PRESENT | Flags::READABLE_WRITABLE;
+        Descriptor(flags.bits())
+    }
+
+    /// Creates a segment descriptor for a protected mode ring 3 data segment.
+    #[inline]
+    pub fn user_data_segment() -> Descriptor {
+        use self::DescriptorFlags as Flags;
+
+        let flags =
+            Flags::USER_SEGMENT | Flags::PRESENT | Flags::READABLE_WRITABLE | Flags::DPL_RING_3;
+        Descriptor(flags.bits())
+    }
+
+    /// Creates a segment descriptor for a protected mode ring 3 code segment.
+    #[inline]
+    pub fn user_code_segment() -> Descriptor {
+        use self::DescriptorFlags as Flags;
+
+        let flags = Flags::USER_SEGMENT
+            | Flags::PRESENT
+            | Flags::EXECUTABLE
+            | Flags::DPL_RING_3
+            | Flags::READABLE_WRITABLE;
+        Descriptor(flags.bits())
+    }
+
+    /// Creates a TSS system descriptor for the given TSS.
+    #[inline]
+    pub fn tss_segment(tss: &'static TaskStateSegment) -> Descriptor {
+        use self::DescriptorFlags as Flags;
+        use core::mem::size_of;
+
+        let ptr = tss as *const _ as u64;
+
+        let mut val = Flags::PRESENT.bits();
+        // base
+        val.set_bits(16..40, ptr.get_bits(0..24));
+        val.set_bits(56..64, ptr.get_bits(24..32));
+        // limit (the `-1` in needed since the bound is inclusive)
+        val.set_bits(0..16, (size_of::<TaskStateSegment>() - 1) as u64);
+        // type (0b1001 = available 32-bit tss)
+        val.set_bits(40..44, 0b1001);
+
+        Descriptor(val)
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+#[repr(C, packed)]
+pub struct TaskStateSegment {
+    /// Used for hardware task switching
+    prev_tss: u32,
+    /// The full 64-bit canonical forms of the stack pointers (RSP) for privilege levels 0-2.
+    pub privilege_stack_table: [Stack; 3],
+
+    cr3: u32,
+    eip: u32,
+    eflags: u32,
+    eax: u32,
+    ecx: u32,
+    edx: u32,
+    ebx: u32,
+    esp: u32,
+    ebp: u32,
+    esi: u32,
+    edi: u32,
+    es: u32,
+    cs: u32,
+    ss: u32,
+    ds: u32,
+    fs: u32,
+    gs: u32,
+    ldt: u32,
+    trap: u16,
+    pub iomap_base: u16,
+}
+
+impl TaskStateSegment {
+    /// Creates a new TSS with zeroed privilege and interrupt stack table and a zero
+    /// `iomap_base`.
+    #[inline]
+    pub const fn new() -> TaskStateSegment {
+        TaskStateSegment {
+            privilege_stack_table: [Stack::zero(); 3],
+            iomap_base: 0,
+            prev_tss: 0,
+            cr3: 0,
+            eip: 0,
+            eflags: 0,
+            eax: 0,
+            ecx: 0,
+            edx: 0,
+            ebx: 0,
+            esp: 0,
+            ebp: 0,
+            esi: 0,
+            edi: 0,
+            es: 0,
+            cs: 0,
+            ss: 0,
+            ds: 0,
+            fs: 0,
+            gs: 0,
+            ldt: 0,
+            trap: 0,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+#[repr(C, packed)]
+pub struct Stack {
+    esp: u32,
+    ss: u32,
+}
+
+impl Stack {
+    fn zero() -> Self {
+        Stack { esp: 0, ss: 0 }
+    }
+}

src/v86/src/gdt.rs

@@ -3,6 +3,7 @@
 #![feature(llvm_asm)]
 #![no_std]
 
+pub mod gdt;
 pub mod idt;
 
 const EFLAG_IF: u16 = 0x00000200;