1 files changed, 135 insertions, 0 deletions
diff --git a/src/shared/segmentation.rs b/src/shared/segmentation.rs
new file mode 100644
index 0000000..f4dc194
--- /dev/null
+++ b/src/shared/segmentation.rs
@@ -0,0 +1,135 @@
+use core::fmt;
+
+use shared::PrivilegeLevel;
+
+/// Specifies which element to load into a segment from
+/// descriptor tables (i.e., is a index to LDT or GDT table
+/// with some additional flags).
+bitflags! {
+    #[repr(C, packed)]
+    pub flags SegmentSelector: u16 {
+        /// Requestor Privilege Level
+        const RPL_0 = 0b00,
+        const RPL_1 = 0b01,
+        const RPL_2 = 0b10,
+        const RPL_3 = 0b11,
+
+        /// Table Indicator (TI) 0 means GDT is used.
+        const TI_GDT = 0 << 3,
+        /// Table Indicator (TI) 1 means LDT is used.
+        const TI_LDT = 1 << 3,
+    }
+}
+
+/// Reload code segment register.
+/// Note this is special since we can not directly move
+/// to %cs. Instead we push the new segment selector
+/// and return value on the stack and use lretq
+/// to reload cs and continue at 1:.
+pub unsafe fn set_cs(sel: SegmentSelector) {
+
+    #[cfg(target_arch="x86")]
+    #[inline(always)]
+    unsafe fn inner(sel: SegmentSelector) {
+        asm!("pushl $0; \
+              pushl $$1f; \
+              lretl; \
+              1:" :: "ri" (sel.bits() as usize) : "{rax}" "memory");
+    }
+
+    #[cfg(target_arch="x86_64")]
+    #[inline(always)]
+    unsafe fn inner(sel: SegmentSelector) {
+        asm!("pushq $0; \
+              leaq  1f(%rip), %rax; \
+              pushq %rax; \
+              lretq; \
+              1:" :: "ri" (sel.bits() as usize) : "{rax}" "memory");
+    }
+
+    inner(sel)
+}
+
+
+impl SegmentSelector {
+    /// Create a new SegmentSelector
+    ///
+    /// # Arguments
+    ///  * `index` index in GDT or LDT array.
+    ///
+    pub const fn new(index: u16, rpl: PrivilegeLevel) -> SegmentSelector {
+        SegmentSelector { bits: index << 3 | (rpl as u16) }
+    }
+
+    pub const fn from_raw(bits: u16) -> SegmentSelector {
+        SegmentSelector { bits: bits }
+    }
+}
+
+impl fmt::Display for SegmentSelector {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let r0 = match self.contains(RPL_0) {
+            false => "",
+            true => "Ring 0 segment selector.",
+        };
+        let r1 = match self.contains(RPL_1) {
+            false => "",
+            true => "Ring 1 segment selector.",
+        };
+        let r2 = match self.contains(RPL_2) {
+            false => "",
+            true => "Ring 2 segment selector.",
+        };
+        let r3 = match self.contains(RPL_3) {
+            false => "",
+            true => "Ring 3 segment selector.",
+        };
+        let tbl = match self.contains(TI_LDT) {
+            false => "GDT Table",
+            true => "LDT Table",
+        };
+
+        write!(f,
+               "Index {} in {}, {}{}{}{}",
+               self.bits >> 3,
+               tbl,
+               r0,
+               r1,
+               r2,
+               r3)
+        // write!(f, "Index")
+    }
+}
+
+
+/// Reload stack segment register.
+pub unsafe fn load_ss(sel: SegmentSelector) {
+    asm!("movw $0, %ss " :: "r" (sel) : "memory");
+}
+
+/// Reload data segment register.
+pub unsafe fn load_ds(sel: SegmentSelector) {
+    asm!("movw $0, %ds " :: "r" (sel) : "memory");
+}
+
+/// Reload es segment register.
+pub unsafe fn load_es(sel: SegmentSelector) {
+    asm!("movw $0, %es " :: "r" (sel) : "memory");
+}
+
+/// Reload fs segment register.
+pub unsafe fn load_fs(sel: SegmentSelector) {
+    asm!("movw $0, %fs " :: "r" (sel) : "memory");
+}
+
+/// Reload gs segment register.
+pub unsafe fn load_gs(sel: SegmentSelector) {
+    asm!("movw $0, %gs " :: "r" (sel) : "memory");
+}
+
+/// Returns the current value of the code segment register.
+pub fn cs() -> SegmentSelector {
+    let segment: u16;
+    unsafe { asm!("mov %cs, $0" : "=r" (segment) ) };
+    SegmentSelector::from_raw(segment)
+}