gdt: Add remaining GDT flags

Add the remaining GDT flags. This now allows users to create 32-bit
and 64-bit GDTs.

We also add flag aliases for common cases. We then verify that these
cases match what the Linux kernel uses by default. This also allows us
to make additional methods `const fn`.

I used [AMD64 Architecture Programmer’s Manual, Volume 2: System Programming](https://www.amd.com/system/files/TechDocs/24593.pdf)
as the main reference for this PR.

Two remaining open questions:
  - Should we set `WRITABLE` (aka Readable for code-segments) by default?
    - This bit is ignored in Long Mode (for code and data)
    - Linux always sets it by default
    - This is probably what users want for legacy mode segments.
  - Should we set `ACCESSED` by default?
    - Linux [sets it by default](https://github.com/torvalds/linux/blob/00e4db51259a5f936fec1424b884f029479d3981/arch/x86/kernel/cpu/common.c#L124-L129)
    - Except [sometimes it doesn't](https://github.com/torvalds/linux/blob/ab851d49f6bfc781edd8bd44c72ec1e49211670b/arch/x86/platform/pvh/head.S#L160-L166)
    - AMD claims this is ignored, but Intel doesn't make the same claim

Signed-off-by: Joe Richey <joerichey@google.com>

src/structures/gdt.rs

@@ -184,55 +184,80 @@ pub enum Descriptor {
 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. Ignored for code segments.
+        /// Set by the processor if this segment has been accessed. Only cleared by software.
+        const ACCESSED          = 1 << 40;
+        /// For 32-bit data segments, sets the segment as writable. For 32-bit code segments,
+        /// sets the segment as _readable_. In 64-bit mode, ignored for all segments.
         const WRITABLE          = 1 << 41;
-        /// Marks a code segment as “conforming”. This influences the privilege checks that
-        /// occur on control transfers.
+        /// For code segments, sets the segment as “conforming”, influencing the
+        /// privilege checks that occur on control transfers. For 32-bit data segments,
+        /// sets the segment as "expand down". In 64-bit mode, ignored for data segments.
         const CONFORMING        = 1 << 42;
-        /// This flag must be set for code segments.
+        /// This flag must be set for code segments and unset for data segments.
         const EXECUTABLE        = 1 << 43;
         /// This flag must be set for user segments (in contrast to system segments).
         const USER_SEGMENT      = 1 << 44;
+        /// The DPL for this descriptor is Ring 3. In 64-bit mode, ignored for data segments.
+        const DPL_RING_3        = 3 << 45;
         /// 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.
+        /// Available for use by the Operating System
+        const AVAILABLE         = 1 << 52;
+        /// Must be set for 64-bit code segments, unset otherwise.
         const LONG_MODE         = 1 << 53;
+        /// Use 32-bit (as opposed to 16-bit) operands. If [`LONG_MODE`] is set,
+        /// this must be unset. In 64-bit mode, ignored for data segments.
+        const DEFAULT_SIZE      = 1 << 54;
+        /// Limit field is scaled by 4096 bytes. In 64-bit mode, ignored for all segments.
+        const GRANULARITY       = 1 << 55;
 
-        /// The DPL for this descriptor is Ring 3
-        const DPL_RING_3        = 3 << 45;
+        /// Bits 0..=15 of the limit field (ignored in 64-bit mode)
+        const LIMIT_0_15        = 0xFFFF;
+        /// Bits 16..=19 of the limit field (ignored in 64-bit mode)
+        const LIMIT_16_19       = 0xF << 48;
+        /// Bits 0..=23 of the base field (ignored in 64-bit mode)
+        const BASE_0_23         = 0xFF_FFFF << 16;
+        /// Bits 24..=31 of the base field (ignored in 64-bit mode)
+        const BASE_24_31        = 0xFF << 56;
+
+        /// Flags that should be set for all flat user segments
+        const FLAT_COMMON = Self::LIMIT_0_15.bits | Self::LIMIT_16_19.bits
+            | Self::GRANULARITY.bits | Self::ACCESSED.bits | Self::WRITABLE.bits
+            | Self::USER_SEGMENT.bits | Self::PRESENT.bits;
+
+        /// Flags for a flat 32-bit kernel code segment
+        const KERNEL_CODE32 = Self::FLAT_COMMON.bits | Self::EXECUTABLE.bits | Self::DEFAULT_SIZE.bits;
+        /// Flags for a 64-bit kernel code segment
+        const KERNEL_CODE64 = Self::FLAT_COMMON.bits | Self::EXECUTABLE.bits | Self::LONG_MODE.bits;
+        /// Flags for a kernel data segment (64-bit or flat 32-bit)
+        const KERNEL_DATA = Self::FLAT_COMMON.bits | Self::DEFAULT_SIZE.bits;
+
+        /// Flags for a flat 32-bit user code segment
+        const USER_CODE32 = Self::KERNEL_CODE32.bits | Self::DPL_RING_3.bits;
+        /// Flags for a 64-bit user code segment
+        const USER_CODE64 = Self::KERNEL_CODE64.bits | Self::DPL_RING_3.bits;
+        /// Flags for a user data segment (64-bit or flat 32-bit)
+        const USER_DATA = Self::KERNEL_DATA.bits | Self::DPL_RING_3.bits;
     }
 }
 
 impl Descriptor {
-    /// Creates a segment descriptor for a long mode kernel code segment.
+    /// Creates a segment descriptor for a 64-bit 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::LONG_MODE;
-        Descriptor::UserSegment(flags.bits())
+    pub const fn kernel_code_segment() -> Descriptor {
+        Descriptor::UserSegment(DescriptorFlags::KERNEL_CODE64.bits())
     }
 
-    /// Creates a segment descriptor for a long mode ring 3 data segment.
+    /// Creates a segment descriptor for a ring 3 data segment (64-bit or 32-bit).
     #[inline]
-    pub fn user_data_segment() -> Descriptor {
-        use self::DescriptorFlags as Flags;
-
-        let flags = Flags::USER_SEGMENT | Flags::PRESENT | Flags::WRITABLE | Flags::DPL_RING_3;
-        Descriptor::UserSegment(flags.bits())
+    pub const fn user_data_segment() -> Descriptor {
+        Descriptor::UserSegment(DescriptorFlags::USER_DATA.bits())
     }
 
-    /// Creates a segment descriptor for a long mode ring 3 code segment.
+    /// Creates a segment descriptor for a 64-bit 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::LONG_MODE
-            | Flags::DPL_RING_3;
-        Descriptor::UserSegment(flags.bits())
+    pub const fn user_code_segment() -> Descriptor {
+        Descriptor::UserSegment(DescriptorFlags::USER_CODE64.bits())
     }
 
     /// Creates a TSS system descriptor for the given TSS.
@@ -258,3 +283,21 @@ impl Descriptor {
         Descriptor::SystemSegment(low, high)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::DescriptorFlags as Flags;
+
+    #[test]
+    #[rustfmt::skip]
+    pub fn linux_kernel_defaults() {
+        // Make sure our defaults match the ones used by the Linux kernel.
+        // Constants pulled from an old version of arch/x86/kernel/cpu/common.c
+        assert_eq!(Flags::KERNEL_CODE32.bits(), 0x00cf9b000000ffff);
+        assert_eq!(Flags::KERNEL_CODE64.bits(), 0x00af9b000000ffff);
+        assert_eq!(Flags::KERNEL_DATA.bits(),   0x00cf93000000ffff);
+        assert_eq!(Flags::USER_CODE32.bits(),   0x00cffb000000ffff);
+        assert_eq!(Flags::USER_DATA.bits(),     0x00cff3000000ffff);
+        assert_eq!(Flags::USER_CODE64.bits(),   0x00affb000000ffff);
+    }
+}