hyperlight_guest/exceptions/
gdt.rs

1/*
2Copyright 2024 The Hyperlight Authors.
3
4Licensed under the Apache License, Version 2.0 (the "License");
5you may not use this file except in compliance with the License.
6You may obtain a copy of the License at
7
8    http://www.apache.org/licenses/LICENSE-2.0
9
10Unless required by applicable law or agreed to in writing, software
11distributed under the License is distributed on an "AS IS" BASIS,
12WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13See the License for the specific language governing permissions and
14limitations under the License.
15*/
16
17use core::arch::asm;
18use core::ptr::addr_of;
19
20/// Entry in the Global Descriptor Table (GDT)
21/// For reference, see page 3-10 Vol. 3A of Intel 64 and IA-32
22/// Architectures Software Developer's Manual, figure 3-8
23/// (https://i.imgur.com/1i9xUmx.png).
24/// From the bottom, we have:
25/// - segment limit 15..0 = limit_low
26/// - base address 31..16 = base_low
27/// - base 23..16 = base_middle
28/// - p dpl s type 15..8 = access
29/// - p d/b l avl seg. limit 23..16 = flags_limit
30/// - base 31..24 = base_high
31#[repr(C, align(8))]
32pub struct GdtEntry {
33    limit_low: u16,
34    base_low: u16,
35    base_middle: u8,
36    access: u8,
37    flags_limit: u8,
38    base_high: u8,
39}
40
41impl GdtEntry {
42    /// Creates a new GDT entry.
43    pub const fn new(base: u32, limit: u32, access: u8, flags: u8) -> Self {
44        Self {
45            base_low: (base & 0xffff) as u16,
46            base_middle: ((base >> 16) & 0xff) as u8,
47            base_high: ((base >> 24) & 0xff) as u8,
48            limit_low: (limit & 0xffff) as u16,
49            flags_limit: (((limit >> 16) & 0x0f) as u8) | ((flags & 0x0f) << 4),
50            access,
51        }
52    }
53}
54
55// Global Descriptor Table (GDT)
56// For reference, see page 2-3 Vol. 3A of Intel 64 and IA-32
57// Architectures Software Developer's Manual.
58static mut GDT: [GdtEntry; 3] = [
59    // Null descriptor
60    GdtEntry::new(0, 0, 0, 0),
61    // Kernel Code Segment (0x08)
62    GdtEntry::new(0, 0, 0x9A, 0xA),
63    // Kernel Data Segment (0x10)
64    GdtEntry::new(0, 0, 0x92, 0xC),
65];
66
67/// GDTR (GDT pointer)
68#[repr(C, packed)]
69struct GdtPointer {
70    size: u16,
71    base: u64,
72}
73
74/// Load the GDT
75pub unsafe fn load_gdt() {
76    let gdt_ptr = GdtPointer {
77        size: (core::mem::size_of::<[GdtEntry; 3]>() - 1) as u16,
78        base: addr_of!(GDT) as *const _ as u64,
79    };
80
81    asm!(
82    "lgdt [{0}]",
83    "mov ax, 0x10",        // Load data segment registers
84    "mov ds, ax",
85    "mov es, ax",
86    "mov fs, ax",
87    "mov gs, ax",
88    "mov ss, ax",
89    "push 0x08",            // Push CS (kernel code segment)
90    "lea rax, [2f + rip]",  // Load the next instruction's address
91    "push rax",             // Push address onto stack
92    "retfq",                // Far return to update CS
93    "2:",                   // Label for continued execution
94    in(reg) &gdt_ptr,
95    options(nostack, preserves_flags)
96    );
97}
hyperlight_guest/exceptions/gdt.rs

hyperlight_guest/exceptions/
gdt.rs
