Move segment types into a new registers::segmentation module

The segment types themselves do not require the `instructions` feature, so it makes sense to move them to the top-level `registers` module. The `instructions::segmentation` module still re-exports all the types, so this change should not break any code.

Fixes #307

.github/workflows/build.yml

@@ -77,6 +77,12 @@ jobs:
         args: --no-default-features --features external_asm,instructions
       if: runner.os != 'Windows'
 
+    - name: "Run cargo doc without default features"
+      uses: actions-rs/cargo@v1
+      with:
+        command: doc
+        args: --no-default-features
+
     - name: "Run cargo build for stable without instructions"
       uses: actions-rs/cargo@v1
       with:

src/instructions/segmentation.rs

@@ -1,64 +1,12 @@
 //! Provides functions to read and write segment registers.
 
-#[cfg(doc)]
-use crate::{
-    registers::control::Cr4Flags,
-    structures::gdt::{Descriptor, GlobalDescriptorTable},
-};
+pub use crate::registers::segmentation::{Segment, Segment64, CS, DS, ES, FS, GS, SS};
 use crate::{
     registers::model_specific::{FsBase, GsBase, Msr},
     structures::gdt::SegmentSelector,
     VirtAddr,
 };
 
-/// An x86 segment
-///
-/// Segment registers on x86 are 16-bit [`SegmentSelector`]s, which index into
-/// the [`GlobalDescriptorTable`]. The corresponding GDT entry is used to
-/// configure the segment itself. Note that most segmentation functionality is
-/// disabled in 64-bit mode. See the individual segments for more information.
-pub trait Segment {
-    /// Returns the current value of the segment register.
-    fn get_reg() -> SegmentSelector;
-    /// Reload the segment register. Depending on the segment, this may also
-    /// reconfigure the corresponding segment.
-    ///
-    /// ## Safety
-    ///
-    /// This function is unsafe because the caller must ensure that `sel`
-    /// is a valid segment descriptor, and that reconfiguring the segment will
-    /// not cause undefined behavior.
-    unsafe fn set_reg(sel: SegmentSelector);
-}
-
-/// An x86 segment which is actually used in 64-bit mode
-///
-/// While most segments are unused in 64-bit mode, the FS and GS segments are
-/// still partially used. Only the 64-bit segment base address is used, and this
-/// address can be set via the GDT, or by using the `FSGSBASE` instructions.
-pub trait Segment64: Segment {
-    /// MSR containing the segment base. This MSR can be used to set the base
-    /// when [`CR4.FSGSBASE`][Cr4Flags::FSGSBASE] is **not** set.
-    const BASE: Msr;
-    /// Reads the segment base address
-    ///
-    /// ## Exceptions
-    ///
-    /// If [`CR4.FSGSBASE`][Cr4Flags::FSGSBASE] is not set, this instruction will throw a `#UD`.
-    fn read_base() -> VirtAddr;
-    /// Writes the segment base address
-    ///
-    /// ## Exceptions
-    ///
-    /// If [`CR4.FSGSBASE`][Cr4Flags::FSGSBASE] is not set, this instruction will throw a `#UD`.
-    ///
-    /// ## Safety
-    ///
-    /// The caller must ensure that this write operation has no unsafe side
-    /// effects, as the segment base address might be in use.
-    unsafe fn write_base(base: VirtAddr);
-}
-
 macro_rules! get_reg_impl {
     ($name:literal, $asm_get:ident) => {
         fn get_reg() -> SegmentSelector {
@@ -120,14 +68,6 @@ macro_rules! segment64_impl {
     };
 }
 
-/// Code Segment
-///
-/// The segment base and limit are unused in 64-bit mode. Only the L (long), D
-/// (default operation size), and DPL (descriptor privilege-level) fields of the
-/// descriptor are recognized. So changing the segment register can be used to
-/// change privilege level or enable/disable long mode.
-#[derive(Debug)]
-pub struct CS;
 impl Segment for CS {
     get_reg_impl!("cs", x86_64_asm_get_cs);
 
@@ -157,50 +97,11 @@ impl Segment for CS {
     }
 }
 
-/// Stack Segment
-///
-/// Entirely unused in 64-bit mode; setting the segment register does nothing.
-/// However, in ring 3, the SS register still has to point to a valid
-/// [`Descriptor`] (it cannot be zero). This means a user-mode read/write
-/// segment descriptor must be present in the GDT.
-///
-/// This register is also set by the `syscall`/`sysret` and
-/// `sysenter`/`sysexit` instructions (even on 64-bit transitions). This is to
-/// maintain symmetry with 32-bit transitions where setting SS actually will
-/// actually have an effect.
-#[derive(Debug)]
-pub struct SS;
 segment_impl!(SS, "ss", x86_64_asm_get_ss, x86_64_asm_load_ss);
-
-/// Data Segment
-///
-/// Entirely unused in 64-bit mode; setting the segment register does nothing.
-#[derive(Debug)]
-pub struct DS;
 segment_impl!(DS, "ds", x86_64_asm_get_ds, x86_64_asm_load_ds);
-
-/// ES Segment
-///
-/// Entirely unused in 64-bit mode; setting the segment register does nothing.
-#[derive(Debug)]
-pub struct ES;
 segment_impl!(ES, "es", x86_64_asm_get_es, x86_64_asm_load_es);
-
-/// FS Segment
-///
-/// Only base is used in 64-bit mode, see [`Segment64`]. This is often used in
-/// user-mode for Thread-Local Storage (TLS).
-#[derive(Debug)]
-pub struct FS;
 segment_impl!(FS, "fs", x86_64_asm_get_fs, x86_64_asm_load_fs);
 segment64_impl!(FS, "fs", FsBase, x86_64_asm_rdfsbase, x86_64_asm_wrfsbase);
-
-/// GS Segment
-///
-/// Only base is used in 64-bit mode, see [`Segment64`]. In kernel-mode, the GS
-/// base often points to a per-cpu kernel data structure.
-#[derive(Debug)]
-pub struct GS;
 segment_impl!(GS, "gs", x86_64_asm_get_gs, x86_64_asm_load_gs);
 segment64_impl!(GS, "gs", GsBase, x86_64_asm_rdgsbase, x86_64_asm_wrgsbase);
 

src/registers/control.rs

@@ -1,9 +1,6 @@
 //! Functions to read and write control registers.
 
 pub use super::model_specific::{Efer, EferFlags};
-#[cfg(doc)]
-use crate::{registers::rflags::RFlags, structures::paging::PageTableFlags};
-
 use bitflags::bitflags;
 
 /// Various control flags modifying the basic operation of the CPU.

src/registers/mod.rs

@@ -3,6 +3,7 @@
 pub mod control;
 pub mod model_specific;
 pub mod rflags;
+pub mod segmentation;
 pub mod xcontrol;
 
 #[cfg(feature = "instructions")]

src/registers/model_specific.rs

@@ -1,12 +1,9 @@
 //! Functions to read and write model specific registers.
 
-#[cfg(doc)]
-use crate::{
-    instructions::segmentation::{Segment64, FS, GS},
-    registers::control::Cr4Flags,
-};
-
 use bitflags::bitflags;
+// imports for intra doc links
+#[cfg(doc)]
+use crate::registers::segmentation::{FS, GS};
 
 /// A model specific register.
 #[derive(Debug)]
@@ -30,13 +27,19 @@ pub struct FsBase;
 
 /// [GS].Base Model Specific Register.
 ///
-/// [`GS::swap`] swaps this register with [`KernelGsBase`].
+#[cfg_attr(
+    feature = "instructions",
+    doc = "[`GS::swap`] swaps this register with [`KernelGsBase`]."
+)]
 #[derive(Debug)]
 pub struct GsBase;
 
 /// KernelGsBase Model Specific Register.
 ///
-/// [`GS::swap`] swaps this register with [`GsBase`].
+#[cfg_attr(
+    feature = "instructions",
+    doc = "[`GS::swap`] swaps this register with [`GsBase`]."
+)]
 #[derive(Debug)]
 pub struct KernelGsBase;
 
@@ -118,6 +121,12 @@ mod x86_64 {
     use crate::PrivilegeLevel;
     use bit_field::BitField;
     use core::convert::TryInto;
+    // imports for intra doc links
+    #[cfg(doc)]
+    use crate::registers::{
+        control::Cr4Flags,
+        segmentation::{Segment, Segment64, CS, SS},
+    };
 
     impl Msr {
         /// Read 64 bits msr register.

src/registers/segmentation.rs

@@ -0,0 +1,155 @@
+//! Abstractions for segment registers.
+
+use super::model_specific::Msr;
+use crate::{PrivilegeLevel, VirtAddr};
+use bit_field::BitField;
+use core::fmt;
+// imports for intra doc links
+#[cfg(doc)]
+use crate::registers::control::Cr4Flags;
+
+/// An x86 segment
+///
+/// Segment registers on x86 are 16-bit [`SegmentSelector`]s, which index into
+/// the [`GlobalDescriptorTable`](crate::structures::gdt::GlobalDescriptorTable). The
+/// corresponding GDT entry is used to
+/// configure the segment itself. Note that most segmentation functionality is
+/// disabled in 64-bit mode. See the individual segments for more information.
+pub trait Segment {
+    /// Returns the current value of the segment register.
+    fn get_reg() -> SegmentSelector;
+    /// Reload the segment register. Depending on the segment, this may also
+    /// reconfigure the corresponding segment.
+    ///
+    /// ## Safety
+    ///
+    /// This function is unsafe because the caller must ensure that `sel`
+    /// is a valid segment descriptor, and that reconfiguring the segment will
+    /// not cause undefined behavior.
+    unsafe fn set_reg(sel: SegmentSelector);
+}
+
+/// An x86 segment which is actually used in 64-bit mode
+///
+/// While most segments are unused in 64-bit mode, the FS and GS segments are
+/// still partially used. Only the 64-bit segment base address is used, and this
+/// address can be set via the GDT, or by using the `FSGSBASE` instructions.
+pub trait Segment64: Segment {
+    /// MSR containing the segment base. This MSR can be used to set the base
+    /// when [`CR4.FSGSBASE`][Cr4Flags::FSGSBASE] is **not** set.
+    const BASE: Msr;
+    /// Reads the segment base address
+    ///
+    /// ## Exceptions
+    ///
+    /// If [`CR4.FSGSBASE`][Cr4Flags::FSGSBASE] is not set, this instruction will throw a `#UD`.
+    fn read_base() -> VirtAddr;
+    /// Writes the segment base address
+    ///
+    /// ## Exceptions
+    ///
+    /// If [`CR4.FSGSBASE`][Cr4Flags::FSGSBASE] is not set, this instruction will throw a `#UD`.
+    ///
+    /// ## Safety
+    ///
+    /// The caller must ensure that this write operation has no unsafe side
+    /// effects, as the segment base address might be in use.
+    unsafe fn write_base(base: VirtAddr);
+}
+
+/// 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).
+///
+/// See Intel 3a, Section 3.4.2 "Segment Selectors"
+#[derive(Clone, Copy, PartialEq, Eq)]
+#[repr(transparent)]
+pub struct SegmentSelector(pub u16);
+
+impl SegmentSelector {
+    /// Creates a new SegmentSelector
+    ///
+    /// # Arguments
+    ///  * `index`: index in GDT or LDT array (not the offset)
+    ///  * `rpl`: the requested privilege level
+    #[inline]
+    pub const fn new(index: u16, rpl: PrivilegeLevel) -> SegmentSelector {
+        SegmentSelector(index << 3 | (rpl as u16))
+    }
+
+    /// Returns the GDT index.
+    #[inline]
+    pub fn index(self) -> u16 {
+        self.0 >> 3
+    }
+
+    /// Returns the requested privilege level.
+    #[inline]
+    pub fn rpl(self) -> PrivilegeLevel {
+        PrivilegeLevel::from_u16(self.0.get_bits(0..2))
+    }
+
+    /// Set the privilege level for this Segment selector.
+    #[inline]
+    pub fn set_rpl(&mut self, rpl: PrivilegeLevel) {
+        self.0.set_bits(0..2, rpl as u16);
+    }
+}
+
+impl fmt::Debug for SegmentSelector {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let mut s = f.debug_struct("SegmentSelector");
+        s.field("index", &self.index());
+        s.field("rpl", &self.rpl());
+        s.finish()
+    }
+}
+
+/// Code Segment
+///
+/// The segment base and limit are unused in 64-bit mode. Only the L (long), D
+/// (default operation size), and DPL (descriptor privilege-level) fields of the
+/// descriptor are recognized. So changing the segment register can be used to
+/// change privilege level or enable/disable long mode.
+#[derive(Debug)]
+pub struct CS;
+
+/// Stack Segment
+///
+/// Entirely unused in 64-bit mode; setting the segment register does nothing.
+/// However, in ring 3, the SS register still has to point to a valid
+/// [`Descriptor`](crate::structures::gdt::Descriptor) (it cannot be zero). This
+/// means a user-mode read/write segment descriptor must be present in the GDT.
+///
+/// This register is also set by the `syscall`/`sysret` and
+/// `sysenter`/`sysexit` instructions (even on 64-bit transitions). This is to
+/// maintain symmetry with 32-bit transitions where setting SS actually will
+/// actually have an effect.
+#[derive(Debug)]
+pub struct SS;
+
+/// Data Segment
+///
+/// Entirely unused in 64-bit mode; setting the segment register does nothing.
+#[derive(Debug)]
+pub struct DS;
+
+/// ES Segment
+///
+/// Entirely unused in 64-bit mode; setting the segment register does nothing.
+#[derive(Debug)]
+pub struct ES;
+
+/// FS Segment
+///
+/// Only base is used in 64-bit mode, see [`Segment64`]. This is often used in
+/// user-mode for Thread-Local Storage (TLS).
+#[derive(Debug)]
+pub struct FS;
+
+/// GS Segment
+///
+/// Only base is used in 64-bit mode, see [`Segment64`]. In kernel-mode, the GS
+/// base often points to a per-cpu kernel data structure.
+#[derive(Debug)]
+pub struct GS;