rust-raspberrypi-OS-tutorials/11_virtual_memory/src/memory/mmu.rs

// SPDX-License-Identifier: MIT OR Apache-2.0
//
// Copyright (c) 2020 Andre Richter <andre.o.richter@gmail.com>

//! Memory Management Unit.
//!
//! In order to decouple `BSP` and `arch` parts of the MMU code (to keep them pluggable), this file
//! provides types for composing an architecture-agnostic description of the kernel 's virtual
//! memory layout.
//!
//! The `BSP` provides such a description through the `bsp::memory::mmu::virt_mem_layout()`
//! function.
//!
//! The `MMU` driver of the `arch` code uses `bsp::memory::mmu::virt_mem_layout()` to compile and
//! install respective page tables.

#[cfg(target_arch = "aarch64")]
#[path = "../_arch/aarch64/memory/mmu.rs"]
mod arch_mmu;
pub use arch_mmu::*;

use core::{fmt, ops::RangeInclusive};

//--------------------------------------------------------------------------------------------------
// Public Definitions
//--------------------------------------------------------------------------------------------------

/// Memory Management interfaces.
pub mod interface {

    /// MMU functions.
    pub trait MMU {
        /// Called by the kernel during early init. Supposed to take the page tables from the
        /// `BSP`-supplied `virt_mem_layout()` and install/activate them for the respective MMU.
        ///
        /// # Safety
        ///
        /// - Changes the HW's global state.
        unsafe fn init(&self) -> Result<(), &'static str>;
    }
}

/// Architecture agnostic translation types.
#[allow(missing_docs)]
#[derive(Copy, Clone)]
pub enum Translation {
    Identity,
    Offset(usize),
}

/// Architecture agnostic memory attributes.
#[allow(missing_docs)]
#[derive(Copy, Clone)]
pub enum MemAttributes {
    CacheableDRAM,
    Device,
}

/// Architecture agnostic access permissions.
#[allow(missing_docs)]
#[derive(Copy, Clone)]
pub enum AccessPermissions {
    ReadOnly,
    ReadWrite,
}

/// Collection of memory attributes.
#[allow(missing_docs)]
#[derive(Copy, Clone)]
pub struct AttributeFields {
    pub mem_attributes: MemAttributes,
    pub acc_perms: AccessPermissions,
    pub execute_never: bool,
}

/// Architecture agnostic descriptor for a memory range.
#[allow(missing_docs)]
pub struct RangeDescriptor {
    pub name: &'static str,
    pub virtual_range: fn() -> RangeInclusive<usize>,
    pub translation: Translation,
    pub attribute_fields: AttributeFields,
}

/// Type for expressing the kernel's virtual memory layout.
pub struct KernelVirtualLayout<const NUM_SPECIAL_RANGES: usize> {
    /// The last (inclusive) address of the address space.
    max_virt_addr_inclusive: usize,

    /// Array of descriptors for non-standard (normal cacheable DRAM) memory regions.
    inner: [RangeDescriptor; NUM_SPECIAL_RANGES],
}

//--------------------------------------------------------------------------------------------------
// Public Code
//--------------------------------------------------------------------------------------------------

impl Default for AttributeFields {
    fn default() -> AttributeFields {
        AttributeFields {
            mem_attributes: MemAttributes::CacheableDRAM,
            acc_perms: AccessPermissions::ReadWrite,
            execute_never: true,
        }
    }
}

/// Human-readable output of a RangeDescriptor.
impl fmt::Display for RangeDescriptor {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // Call the function to which self.range points, and dereference the result, which causes
        // Rust to copy the value.
        let start = *(self.virtual_range)().start();
        let end = *(self.virtual_range)().end();
        let size = end - start + 1;

        // log2(1024).
        const KIB_RSHIFT: u32 = 10;

        // log2(1024 * 1024).
        const MIB_RSHIFT: u32 = 20;

        let (size, unit) = if (size >> MIB_RSHIFT) > 0 {
            (size >> MIB_RSHIFT, "MiB")
        } else if (size >> KIB_RSHIFT) > 0 {
            (size >> KIB_RSHIFT, "KiB")
        } else {
            (size, "Byte")
        };

        let attr = match self.attribute_fields.mem_attributes {
            MemAttributes::CacheableDRAM => "C",
            MemAttributes::Device => "Dev",
        };

        let acc_p = match self.attribute_fields.acc_perms {
            AccessPermissions::ReadOnly => "RO",
            AccessPermissions::ReadWrite => "RW",
        };

        let xn = if self.attribute_fields.execute_never {
            "PXN"
        } else {
            "PX"
        };

        write!(
            f,
            "      {:#010x} - {:#010x} | {: >3} {} | {: <3} {} {: <3} | {}",
            start, end, size, unit, attr, acc_p, xn, self.name
        )
    }
}

impl<const NUM_SPECIAL_RANGES: usize> KernelVirtualLayout<{ NUM_SPECIAL_RANGES }> {
    /// Create a new instance.
    pub const fn new(max: usize, layout: [RangeDescriptor; NUM_SPECIAL_RANGES]) -> Self {
        Self {
            max_virt_addr_inclusive: max,
            inner: layout,
        }
    }

    /// For a virtual address, find and return the output address and corresponding attributes.
    ///
    /// If the address is not found in `inner`, return an identity mapped default with normal
    /// cacheable DRAM attributes.
    pub fn get_virt_addr_properties(
        &self,
        virt_addr: usize,
    ) -> Result<(usize, AttributeFields), &'static str> {
        if virt_addr > self.max_virt_addr_inclusive {
            return Err("Address out of range");
        }

        for i in self.inner.iter() {
            if (i.virtual_range)().contains(&virt_addr) {
                let output_addr = match i.translation {
                    Translation::Identity => virt_addr,
                    Translation::Offset(a) => a + (virt_addr - (i.virtual_range)().start()),
                };

                return Ok((output_addr, i.attribute_fields));
            }
        }

        Ok((virt_addr, AttributeFields::default()))
    }

    /// Print the memory layout.
    pub fn print_layout(&self) {
        use crate::info;

        for i in self.inner.iter() {
            info!("{}", i);
        }
    }
}