cloud-hypervisor/hypervisor/src/arch/x86/gdt.rs

// Copyright © 2020, Oracle and/or its affiliates.
//
// Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0
//
// Portions Copyright 2017 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE-BSD-3-Clause file.

// For GDT details see arch/x86/include/asm/segment.h
use crate::x86_64::SegmentRegister;

/// Constructor for a conventional segment GDT (or LDT) entry. Derived from the kernel's segment.h.
pub fn gdt_entry(flags: u16, base: u32, limit: u32) -> u64 {
    (((base as u64) & 0xff000000u64) << (56 - 24))
        | (((flags as u64) & 0x0000f0ffu64) << 40)
        | (((limit as u64) & 0x000f0000u64) << (48 - 16))
        | (((base as u64) & 0x00ffffffu64) << 16)
        | ((limit as u64) & 0x0000ffffu64)
}

fn get_base(entry: u64) -> u64 {
    (((entry) & 0xFF00000000000000) >> 32)
        | (((entry) & 0x000000FF00000000) >> 16)
        | (((entry) & 0x00000000FFFF0000) >> 16)
}

// Extract the segment limit from the GDT segment descriptor.
//
// In a segment descriptor, the limit field is 20 bits, so it can directly describe
// a range from 0 to 0xFFFFF (1MByte). When G flag is set (4-KByte page granularity) it
// scales the value in the limit field by a factor of 2^12 (4Kbytes), making the effective
// limit range from 0xFFF (4 KBytes) to 0xFFFF_FFFF (4 GBytes).
//
// However, the limit field in the VMCS definition is a 32 bit field, and the limit value is not
// automatically scaled using the G flag. This means that for a desired range of 4GB for a
// given segment, its limit must be specified as 0xFFFF_FFFF. Therefore the method of obtaining
// the limit from the GDT entry is not sufficient, since it only provides 20 bits when 32 bits
// are necessary. Fortunately, we can check if the G flag is set when extracting the limit since
// the full GDT entry is passed as an argument, and perform the scaling of the limit value to
// return the full 32 bit value.
//
// The scaling mentioned above is required when using PVH boot, since the guest boots in protected
// (32-bit) mode and must be able to access the entire 32-bit address space. It does not cause issues
// for the case of direct boot to 64-bit (long) mode, since in 64-bit mode the processor does not
// perform runtime limit checking on code or data segments.
fn get_limit(entry: u64) -> u32 {
    let limit: u32 =
        ((((entry) & 0x000F000000000000) >> 32) | ((entry) & 0x000000000000FFFF)) as u32;

    // Perform manual limit scaling if G flag is set
    match get_g(entry) {
        0 => limit,
        _ => ((limit << 12) | 0xFFF), // G flag is either 0 or 1
    }
}

fn get_g(entry: u64) -> u8 {
    ((entry & 0x0080000000000000) >> 55) as u8
}

fn get_db(entry: u64) -> u8 {
    ((entry & 0x0040000000000000) >> 54) as u8
}

fn get_l(entry: u64) -> u8 {
    ((entry & 0x0020000000000000) >> 53) as u8
}

fn get_avl(entry: u64) -> u8 {
    ((entry & 0x0010000000000000) >> 52) as u8
}

fn get_p(entry: u64) -> u8 {
    ((entry & 0x0000800000000000) >> 47) as u8
}

fn get_dpl(entry: u64) -> u8 {
    ((entry & 0x0000600000000000) >> 45) as u8
}

fn get_s(entry: u64) -> u8 {
    ((entry & 0x0000100000000000) >> 44) as u8
}

fn get_type(entry: u64) -> u8 {
    ((entry & 0x00000F0000000000) >> 40) as u8
}

/// Automatically build the struct for SET_SREGS from the kernel bit fields.
///
/// # Arguments
///
/// * `entry` - The gdt entry.
/// * `table_index` - Index of the entry in the gdt table.
pub fn segment_from_gdt(entry: u64, table_index: u8) -> SegmentRegister {
    SegmentRegister {
        base: get_base(entry),
        limit: get_limit(entry),
        selector: (table_index * 8) as u16,
        type_: get_type(entry),
        present: get_p(entry),
        dpl: get_dpl(entry),
        db: get_db(entry),
        s: get_s(entry),
        l: get_l(entry),
        g: get_g(entry),
        avl: get_avl(entry),
        padding: 0,
        unusable: match get_p(entry) {
            0 => 1,
            _ => 0,
        },
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn field_parse() {
        let gdt = gdt_entry(0xA09B, 0x100000, 0xfffff);
        let seg = segment_from_gdt(gdt, 0);
        // 0xA09B
        // 'A'
        assert_eq!(0x1, seg.g);
        assert_eq!(0x0, seg.db);
        assert_eq!(0x1, seg.l);
        assert_eq!(0x0, seg.avl);
        // '9'
        assert_eq!(0x1, seg.present);
        assert_eq!(0x0, seg.dpl);
        assert_eq!(0x1, seg.s);
        // 'B'
        assert_eq!(0xB, seg.type_);
        // base and limit
        assert_eq!(0x100000, seg.base);
        assert_eq!(0xffffffff, seg.limit);
        assert_eq!(0x0, seg.unusable);
    }
}