ladybird/Kernel/Arch/x86/DescriptorTable.h

/*
 * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
 * Copyright (c) 2021, Leon Albrecht <leon2002.la@gmail.com>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#pragma once

#include <AK/Types.h>
#include <Kernel/VirtualAddress.h>

#define GDT_SELECTOR_CODE0 0x08
#define GDT_SELECTOR_DATA0 0x10
#define GDT_SELECTOR_CODE3 0x18
#define GDT_SELECTOR_DATA3 0x20
#define GDT_SELECTOR_TLS 0x28
#define GDT_SELECTOR_PROC 0x30
#define GDT_SELECTOR_TSS 0x38

// SYSENTER makes certain assumptions on how the GDT is structured:
static_assert(GDT_SELECTOR_CODE0 + 8 == GDT_SELECTOR_DATA0); // SS0 = CS0 + 8

// SYSEXIT makes certain assumptions on how the GDT is structured:
static_assert(GDT_SELECTOR_CODE0 + 16 == GDT_SELECTOR_CODE3); // CS3 = CS0 + 16
static_assert(GDT_SELECTOR_CODE0 + 24 == GDT_SELECTOR_DATA3); // SS3 = CS0 + 32

namespace Kernel {

struct [[gnu::packed]] DescriptorTablePointer {
    u16 limit;
    void* address;
};

union [[gnu::packed]] Descriptor {
    struct {
        u16 limit_lo;
        u16 base_lo;
        u8 base_hi;
        u8 type : 4;
        u8 descriptor_type : 1;
        u8 dpl : 2;
        u8 segment_present : 1;
        u8 limit_hi : 4;
        u8 : 1;
        u8 operation_size64 : 1;
        u8 operation_size32 : 1;
        u8 granularity : 1;
        u8 base_hi2;
    };
    struct {
        u32 low;
        u32 high;
    };

    enum Type {
        Invalid = 0,
        AvailableTSS_16bit = 0x1,
        LDT = 0x2,
        BusyTSS_16bit = 0x3,
        CallGate_16bit = 0x4,
        TaskGate = 0x5,
        InterruptGate_16bit = 0x6,
        TrapGate_16bit = 0x7,
        AvailableTSS_32bit = 0x9,
        BusyTSS_32bit = 0xb,
        CallGate_32bit = 0xc,
        InterruptGate_32bit = 0xe,
        TrapGate_32bit = 0xf,
    };

    VirtualAddress base() const
    {
        FlatPtr base = base_lo;
        base |= base_hi << 16u;
        base |= base_hi2 << 24u;
        return VirtualAddress { base };
    }

    void set_base(VirtualAddress base)
    {
        base_lo = base.get() & 0xffffu;
        base_hi = (base.get() >> 16u) & 0xffu;
        base_hi2 = (base.get() >> 24u) & 0xffu;
    }

    void set_limit(u32 length)
    {
        limit_lo = length & 0xffff;
        limit_hi = (length >> 16) & 0xf;
    }
};

enum class IDTEntryType {
    TaskGate32 = 0b0101,
    InterruptGate16 = 0b110,
    TrapGate16 = 0b111,
    InterruptGate32 = 0b1110,
    TrapGate32 = 0b1111,
};

// Clang doesn't format this right due to the compiler magic
// clang-format off
struct [[gnu::packed]] IDTEntry
{
    u16 offset_1; // offset bits 0..15
    u16 selector; // a code segment selector in GDT or LDT

    u8 zero; // unused, set to 0 (maybe used on amd64)
    struct {
        // FIXME: Is the order correct?
        u8 gate_type : 4;
        u8 storage_segment : 1;
        u8 descriptor_privilege_level : 2;
        u8 present : 1;
    } type_attr;  // type and attributes
    u16 offset_2; // offset bits 16..31
#if !ARCH(I386)
// we may need to switch those around?
    u32 offset_3;
    u32 zeros;
#endif

    IDTEntry() = default;
    IDTEntry(FlatPtr callback, u16 selector_, IDTEntryType type, u8 storage_segment, u8 privilige_level)
        : offset_1 { (u16)((FlatPtr)callback & 0xFFFF) }
        , selector { selector_ }
        , zero { 0 }
        , type_attr {
            .gate_type = (u8)type,
            .storage_segment = storage_segment,
            .descriptor_privilege_level = (u8)(privilige_level & 0b11),
            .present = 1,
        }
        , offset_2 { (u16)((FlatPtr)callback >> 16) }
#if !ARCH(I386)
        , offset_3 { (u32)(((FlatPtr)callback) >> 32) }
        , zeros { 0 }
#endif
    {
    }

    u32 off()
    {
#if ARCH(I386)
        return (u32)offset_2 << 16 & (u32)offset_1;
#else
        return (u64)offset_3 << 32 & (u64)offset_2 << 16 & (u64)offset_1;
#endif
    }
    IDTEntryType type()
    {
        return IDTEntryType(type_attr.gate_type);
    }
};
// clang-format on

}