holey-bytes/hbvm/src/mem/softpaged/paging.rs

//! Page table and associated structures implementation

use core::{fmt::Debug, mem::MaybeUninit};

/// Page entry permission
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
#[repr(u8)]
pub enum Permission {
    /// No page present
    #[default]
    Empty,
    /// Points to another pagetable
    Node,
    /// Page is read only
    Readonly,
    /// Page is readable and writable
    Write,
    /// Page is readable and executable
    Exec,
}

/// Page table entry
#[derive(Clone, Copy, Default, PartialEq, Eq)]
pub struct PtEntry(u64);
impl PtEntry {
    /// Create new
    ///
    /// # Safety
    /// - `ptr` has to point to valid data and shall not be deallocated
    ///    troughout the entry lifetime
    #[inline]
    pub unsafe fn new(ptr: *mut PtPointedData, permission: Permission) -> Self {
        Self(ptr as u64 | permission as u64)
    }

    /// Get permission
    #[inline]
    pub fn permission(&self) -> Permission {
        unsafe { core::mem::transmute(self.0 as u8 & 0b111) }
    }

    /// Get pointer to the data (leaf) or next page table (node)
    #[inline]
    pub fn ptr(&self) -> *mut PtPointedData {
        (self.0 & !((1 << 12) - 1)) as _
    }
}

impl Debug for PtEntry {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_struct("PtEntry")
            .field("ptr", &self.ptr())
            .field("permission", &self.permission())
            .finish()
    }
}

/// Page table
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
#[repr(align(4096))]
pub struct PageTable {
    pub childen: u8,
    pub table:   [PtEntry; 256],
}

impl Default for PageTable {
    fn default() -> Self {
        // SAFETY: It's fine, zeroed page table entry is valid (= empty)
        Self {
            childen: 0,
            table:   unsafe { MaybeUninit::zeroed().assume_init() },
        }
    }
}

/// Data page table entry can possibly point to
#[derive(Clone, Copy)]
#[repr(C, align(4096))]
pub union PtPointedData {
    /// Node - next page table
    pub pt:   PageTable,
    /// Leaf
    pub page: u8,
}