src/mm/mmio_buddy.rs

use crate::libs::spinlock::{SpinLock, SpinLockGuard};
use crate::syscall::SystemError;
use crate::{
    arch::asm::current::current_pcb,
    include::bindings::bindings::{
        initial_mm, mm_create_vma, mm_unmap, vm_area_del, vm_area_free, vm_area_struct, vm_flags_t,
        vma_find, MMIO_BASE, MMIO_TOP, PAGE_1G_SHIFT, PAGE_1G_SIZE, PAGE_2M_SIZE, PAGE_4K_SHIFT,
        PAGE_4K_SIZE, VM_DONTCOPY, VM_IO,
    },
    kdebug, kerror,
};
use alloc::{boxed::Box, collections::LinkedList, vec::Vec};
use core::{mem, ptr::null_mut};

// 最大的伙伴块的幂
const MMIO_BUDDY_MAX_EXP: u32 = PAGE_1G_SHIFT;
// 最小的伙伴块的幂
const MMIO_BUDDY_MIN_EXP: u32 = PAGE_4K_SHIFT;
// 内存池数组的范围
const MMIO_BUDDY_REGION_COUNT: u32 = MMIO_BUDDY_MAX_EXP - MMIO_BUDDY_MIN_EXP + 1;

lazy_static! {
    pub static ref MMIO_POOL: MmioBuddyMemPool = MmioBuddyMemPool::new();
}

pub enum MmioResult {
    SUCCESS,
    EINVAL,
    ENOFOUND,
    WRONGEXP,
    ISEMPTY,
}

/// @brief buddy内存池
pub struct MmioBuddyMemPool {
    pool_start_addr: u64,
    pool_size: u64,
    free_regions: [SpinLock<MmioFreeRegionList>; MMIO_BUDDY_REGION_COUNT as usize],
}
impl Default for MmioBuddyMemPool {
    fn default() -> Self {
        MmioBuddyMemPool {
            pool_start_addr: MMIO_BASE as u64,
            pool_size: (MMIO_TOP - MMIO_BASE) as u64,
            free_regions: unsafe { mem::zeroed() },
        }
    }
}
impl MmioBuddyMemPool {
    fn new() -> Self {
        return MmioBuddyMemPool {
            ..Default::default()
        };
    }

    /// @brief 创建新的地址区域结构体
    ///
    /// @param vaddr 虚拟地址
    ///
    /// @return 创建好的地址区域结构体
    fn create_region(&self, vaddr: u64) -> Box<MmioBuddyAddrRegion> {
        let mut region: Box<MmioBuddyAddrRegion> = Box::new(MmioBuddyAddrRegion::new());
        region.vaddr = vaddr;
        return region;
    }

    /// @brief 将内存块归还给buddy
    ///
    /// @param vaddr 虚拟地址
    ///
    /// @param exp 内存空间的大小（2^exp）
    ///
    /// @param list_guard 【exp】对应的链表
    ///
    /// @return Ok(i32) 返回0
    ///
    /// @return Err(SystemError) 返回错误码
    fn give_back_block(&self, vaddr: u64, exp: u32) -> Result<i32, SystemError> {
        // 确保内存对齐，低位都要为0
        if (vaddr & ((1 << exp) - 1)) != 0 {
            return Err(SystemError::EINVAL);
        }
        let region: Box<MmioBuddyAddrRegion> = self.create_region(vaddr);
        // 加入buddy
        let list_guard: &mut SpinLockGuard<MmioFreeRegionList> =
            &mut self.free_regions[exp2index(exp)].lock();
        self.push_block(region, list_guard);
        return Ok(0);
    }

    /// @brief 将给定大小为2^{exp}的内存块一分为二，并插入内存块大小为2^{exp-1}的链表中
    ///
    /// @param region 要被分割的地址区域结构体（保证其已经从链表中取出）
    ///
    /// @param exp 要被分割的地址区域的大小的幂
    ///
    /// @param list_guard 【exp-1】对应的链表
    fn split_block(
        &self,
        region: Box<MmioBuddyAddrRegion>,
        exp: u32,
        low_list_guard: &mut SpinLockGuard<MmioFreeRegionList>,
    ) {
        let vaddr: u64 = self.calculate_block_vaddr(region.vaddr, exp - 1);
        let new_region: Box<MmioBuddyAddrRegion> = self.create_region(vaddr);
        self.push_block(region, low_list_guard);
        self.push_block(new_region, low_list_guard);
    }

    /// @brief 从buddy中申请一块指定大小的内存区域
    ///
    /// @param exp 要申请的内存块的大小的幂(2^exp)
    ///
    /// @param list_guard exp对应的链表
    ///
    /// @return Ok(Box<MmioBuddyAddrRegion>) 符合要求的内存区域。
    ///
    /// @return Err(MmioResult)
    /// - 没有满足要求的内存块时，返回ENOFOUND
    /// - 申请的内存块大小超过合法范围，返回WRONGEXP
    /// - 调用函数出错时，返回出错函数对应错误码
    fn query_addr_region(
        &self,
        exp: u32,
        list_guard: &mut SpinLockGuard<MmioFreeRegionList>,
    ) -> Result<Box<MmioBuddyAddrRegion>, MmioResult> {
        // 申请范围错误
        if exp < MMIO_BUDDY_MIN_EXP || exp > MMIO_BUDDY_MAX_EXP {
            kdebug!("query_addr_region: exp wrong");
            return Err(MmioResult::WRONGEXP);
        }
        // 没有恰好符合要求的内存块
        // 注意：exp对应的链表list_guard已上锁【注意避免死锁问题】
        if list_guard.num_free == 0 {
            // 找到最小符合申请范围的内存块
            // 将大的内存块依次分成小块内存，直到能够满足exp大小，即将exp+1分成两块exp
            for e in exp + 1..MMIO_BUDDY_MAX_EXP + 1 {
                let pop_list: &mut SpinLockGuard<MmioFreeRegionList> =
                    &mut self.free_regions[exp2index(e) as usize].lock();
                if pop_list.num_free == 0 {
                    continue;
                }

                for e2 in (exp + 1..e + 1).rev() {
                    if e2 == e {
                        match self.pop_block(pop_list) {
                            Ok(region) => {
                                if e2 != exp + 1 {
                                    // 要将分裂后的内存块插入到更小的链表中
                                    let low_list_guard: &mut SpinLockGuard<MmioFreeRegionList> =
                                        &mut self.free_regions[exp2index(e2 - 1) as usize].lock();
                                    self.split_block(region, e2, low_list_guard);
                                } else {
                                    // 由于exp对应的链表list_guard已经被锁住了 不能再加锁
                                    // 所以直接将list_guard传入
                                    self.split_block(region, e2, list_guard);
                                }
                            }
                            Err(err) => {
                                kdebug!("buddy_pop_region get wrong");
                                return Err(err);
                            }
                        }
                    } else {
                        match self.pop_block(&mut self.free_regions[exp2index(e2) as usize].lock())
                        {
                            Ok(region) => {
                                if e2 != exp + 1 {
                                    // 要将分裂后的内存块插入到更小的链表中
                                    let low_list_guard: &mut SpinLockGuard<MmioFreeRegionList> =
                                        &mut self.free_regions[exp2index(e2 - 1) as usize].lock();
                                    self.split_block(region, e2, low_list_guard);
                                } else {
                                    // 由于exp对应的链表list_guard已经被锁住了 不能再加锁
                                    // 所以直接将list_guard传入
                                    self.split_block(region, e2, list_guard);
                                }
                            }
                            Err(err) => {
                                kdebug!("buddy_pop_region get wrong");
                                return Err(err);
                            }
                        }
                    }
                }
                break;
            }
            // 判断是否获得了exp大小的内存块
            if list_guard.num_free > 0 {
                match self.pop_block(list_guard) {
                    Ok(ret) => return Ok(ret),
                    Err(err) => return Err(err),
                }
            }
            // 拆分大内存块无法获得exp大小内存块
            // 尝试用小内存块合成
            // 即将两块exp合成一块exp+1

            // TODO：修改下一个循环的冗余代码，请不要删除此处的注释
            // let merge = |high_list_guard: &mut SpinLockGuard<MmioFreeRegionList>, exp: u32| {
            //     if let Err(err) = self.merge_all_exp(
            //         exp,
            //         &mut self.free_regions[exp2index(exp) as usize].lock(),
            //         high_list_guard,
            //     ) {
            //         return err;
            //     } else {
            //         return MmioResult::SUCCESS;
            //     }
            // };
            for e in MMIO_BUDDY_MIN_EXP..exp {
                if e != exp - 1 {
                    match self.merge_all_exp(
                        exp,
                        &mut self.free_regions[exp2index(exp) as usize].lock(),
                        &mut self.free_regions[exp2index(exp + 1)].lock(),
                    ) {
                        Ok(_) => continue,
                        Err(err) => {
                            kdebug!("merge_all_exp get wrong");
                            return Err(err);
                        }
                    }
                } else {
                    match self.merge_all_exp(
                        exp,
                        &mut self.free_regions[exp2index(exp) as usize].lock(),
                        list_guard,
                    ) {
                        Ok(_) => continue,
                        Err(err) => {
                            kdebug!("merge_all_exp get wrong");
                            return Err(err);
                        }
                    }
                }
            }

            //判断是否获得了exp大小的内存块
            if list_guard.num_free > 0 {
                match self.pop_block(list_guard) {
                    Ok(ret) => return Ok(ret),
                    Err(err) => return Err(err),
                }
            }
            return Err(MmioResult::ENOFOUND);
        } else {
            match self.pop_block(list_guard) {
                Ok(ret) => return Ok(ret),
                Err(err) => return Err(err),
            }
        }
    }

    /// @brief 对query_addr_region进行封装
    ///
    /// @param exp 内存区域的大小(2^exp)
    ///
    /// @return Ok(Box<MmioBuddyAddrRegion>)符合要求的内存块信息结构体。
    /// @return Err(MmioResult) 没有满足要求的内存块时，返回__query_addr_region的错误码。
    fn mmio_buddy_query_addr_region(
        &self,
        exp: u32,
    ) -> Result<Box<MmioBuddyAddrRegion>, MmioResult> {
        let list_guard: &mut SpinLockGuard<MmioFreeRegionList> =
            &mut self.free_regions[exp2index(exp)].lock();
        match self.query_addr_region(exp, list_guard) {
            Ok(ret) => return Ok(ret),
            Err(err) => {
                kdebug!("mmio_buddy_query_addr_region failed");
                return Err(err);
            }
        }
    }
    /// @brief 往指定的地址空间链表中添加一个地址区域
    ///
    /// @param region 要被添加的地址结构体
    ///
    /// @param list_guard 目标链表
    fn push_block(
        &self,
        region: Box<MmioBuddyAddrRegion>,
        list_guard: &mut SpinLockGuard<MmioFreeRegionList>,
    ) {
        list_guard.list.push_back(region);
        list_guard.num_free += 1;
    }

    /// @brief 根据地址和内存块大小，计算伙伴块虚拟内存的地址
    #[inline(always)]
    fn calculate_block_vaddr(&self, vaddr: u64, exp: u32) -> u64 {
        return vaddr ^ (1 << exp);
    }

    /// @brief 寻找并弹出指定内存块的伙伴块
    ///
    /// @param region 对应内存块的信息
    ///
    /// @param exp 内存块大小
    ///
    /// @param list_guard 【exp】对应的链表
    ///
    /// @return Ok(Box<MmioBuddyAddrRegion) 返回伙伴块的引用
    /// @return Err(MmioResult)
    /// - 当链表为空，返回ISEMPTY
    /// - 没有找到伙伴块，返回ENOFOUND
    fn pop_buddy_block(
        &self,
        vaddr: u64,
        exp: u32,
        list_guard: &mut SpinLockGuard<MmioFreeRegionList>,
    ) -> Result<Box<MmioBuddyAddrRegion>, MmioResult> {
        if list_guard.list.len() == 0 {
            return Err(MmioResult::ISEMPTY);
        } else {
            //计算伙伴块的地址
            let buddy_vaddr = self.calculate_block_vaddr(vaddr, exp);

            // element 只会有一个元素
            let mut element: Vec<Box<MmioBuddyAddrRegion>> = list_guard
                .list
                .drain_filter(|x| x.vaddr == buddy_vaddr)
                .collect();
            if element.len() == 1 {
                list_guard.num_free -= 1;
                return Ok(element.pop().unwrap());
            }

            //没有找到对应的伙伴块
            return Err(MmioResult::ENOFOUND);
        }
    }

    /// @brief 从指定空闲链表中取出内存区域
    ///
    /// @param list_guard 【exp】对应的链表
    ///
    /// @return Ok(Box<MmioBuddyAddrRegion>) 内存块信息结构体的引用。
    ///
    /// @return Err(MmioResult) 当链表为空，无法删除时，返回ISEMPTY
    fn pop_block(
        &self,
        list_guard: &mut SpinLockGuard<MmioFreeRegionList>,
    ) -> Result<Box<MmioBuddyAddrRegion>, MmioResult> {
        if !list_guard.list.is_empty() {
            list_guard.num_free -= 1;
            return Ok(list_guard.list.pop_back().unwrap());
        }
        return Err(MmioResult::ISEMPTY);
    }

    /// @brief 合并所有2^{exp}大小的内存块
    ///
    /// @param exp 内存块大小的幂(2^exp)
    ///
    /// @param list_guard exp对应的链表
    ///
    /// @param high_list_guard exp+1对应的链表
    ///
    /// @return Ok(MmioResult) 合并成功返回SUCCESS
    /// @return Err(MmioResult)
    /// - 内存块过少，无法合并，返回EINVAL
    /// - pop_buddy_block调用出错，返回其错误码
    /// - merge_blocks调用出错，返回其错误码
    fn merge_all_exp(
        &self,
        exp: u32,
        list_guard: &mut SpinLockGuard<MmioFreeRegionList>,
        high_list_guard: &mut SpinLockGuard<MmioFreeRegionList>,
    ) -> Result<MmioResult, MmioResult> {
        // 至少要两个内存块才能合并
        if list_guard.num_free <= 1 {
            return Err(MmioResult::EINVAL);
        }
        loop {
            if list_guard.num_free <= 1 {
                break;
            }
            // 获取内存块
            let vaddr: u64 = list_guard.list.back().unwrap().vaddr;
            // 获取伙伴内存块
            match self.pop_buddy_block(vaddr, exp, list_guard) {
                Err(err) => {
                    return Err(err);
                }
                Ok(buddy_region) => {
                    let region: Box<MmioBuddyAddrRegion> = list_guard.list.pop_back().unwrap();
                    let copy_region: Box<MmioBuddyAddrRegion> = Box::new(MmioBuddyAddrRegion {
                        vaddr: region.vaddr,
                    });
                    // 在两块内存都被取出之后才进行合并
                    match self.merge_blocks(region, buddy_region, exp, high_list_guard) {
                        Err(err) => {
                            // 如果合并失败了要将取出来的元素放回去
                            self.push_block(copy_region, list_guard);
                            kdebug!("merge_all_exp: merge_blocks failed");
                            return Err(err);
                        }
                        Ok(_) => continue,
                    }
                }
            }
        }
        return Ok(MmioResult::SUCCESS);
    }

    /// @brief 合并两个【已经从链表中取出】的内存块
    ///
    /// @param region_1 第一个内存块
    ///
    /// @param region_2 第二个内存
    ///
    /// @return Ok(MmioResult) 成功返回SUCCESS
    ///
    /// @return Err(MmioResult) 两个内存块不是伙伴块,返回EINVAL
    fn merge_blocks(
        &self,
        region_1: Box<MmioBuddyAddrRegion>,
        region_2: Box<MmioBuddyAddrRegion>,
        exp: u32,
        high_list_guard: &mut SpinLockGuard<MmioFreeRegionList>,
    ) -> Result<MmioResult, MmioResult> {
        // 判断是否为伙伴块
        if region_1.vaddr != self.calculate_block_vaddr(region_2.vaddr, exp) {
            return Err(MmioResult::EINVAL);
        }
        // 将大的块放进下一级链表
        self.push_block(region_1, high_list_guard);
        return Ok(MmioResult::SUCCESS);
    }

    /// @brief 创建一块mmio区域，并将vma绑定到initial_mm
    ///
    /// @param size mmio区域的大小（字节）
    ///
    /// @param vm_flags 要把vma设置成的标志
    ///
    /// @param res_vaddr 返回值-分配得到的虚拟地址
    ///
    /// @param res_length 返回值-分配的虚拟地址空间长度
    ///
    /// @return Ok(i32) 成功返回0
    ///
    /// @return Err(SystemError) 失败返回错误码
    pub fn create_mmio(
        &self,
        size: u32,
        vm_flags: vm_flags_t,
        res_vaddr: *mut u64,
        res_length: *mut u64,
    ) -> Result<i32, SystemError> {
        if size > PAGE_1G_SIZE || size == 0 {
            return Err(SystemError::EPERM);
        }
        let mut retval: i32 = 0;
        // 计算前导0
        let mut size_exp: u32 = 31 - size.leading_zeros();
        // 记录最终申请的空间大小
        let mut new_size: u32 = size;
        // 对齐要申请的空间大小
        // 如果要申请的空间大小小于4k，则分配4k
        if size_exp < PAGE_4K_SHIFT {
            new_size = PAGE_4K_SIZE;
            size_exp = PAGE_4K_SHIFT;
        } else if (new_size & (!(1 << size_exp))) != 0 {
            // 向左对齐空间大小
            size_exp += 1;
            new_size = 1 << size_exp;
        }
        match MMIO_POOL.mmio_buddy_query_addr_region(size_exp) {
            Ok(region) => {
                unsafe {
                    *res_vaddr = region.vaddr;
                    *res_length = new_size as u64;
                }
                // 创建vma
                let flags: u64 = vm_flags | (VM_IO | VM_DONTCOPY) as u64;
                let len_4k: u64 = (new_size % PAGE_2M_SIZE) as u64;
                let len_2m: u64 = new_size as u64 - len_4k;
                let mut loop_i: u64 = 0;
                // 先分配2M的vma
                loop {
                    if loop_i >= len_2m {
                        break;
                    }
                    let vma: *mut *mut vm_area_struct = null_mut();
                    retval = unsafe {
                        mm_create_vma(
                            &mut initial_mm,
                            region.vaddr + loop_i,
                            PAGE_2M_SIZE.into(),
                            flags,
                            null_mut(),
                            vma,
                        )
                    };
                    if retval != 0 {
                        kdebug!(
                            "failed to create mmio 2m vma. pid = {:?}",
                            current_pcb().pid
                        );
                        unsafe {
                            vm_area_del(*vma);
                            vm_area_free(*vma);
                        }
                        return Err(SystemError::from_posix_errno(retval).unwrap());
                    }
                    loop_i += PAGE_2M_SIZE as u64;
                }
                // 分配4K的vma
                loop_i = len_2m;
                loop {
                    if loop_i >= size as u64 {
                        break;
                    }
                    let vma: *mut *mut vm_area_struct = null_mut();
                    retval = unsafe {
                        mm_create_vma(
                            &mut initial_mm,
                            region.vaddr + loop_i,
                            PAGE_4K_SIZE.into(),
                            flags,
                            null_mut(),
                            vma,
                        )
                    };
                    if retval != 0 {
                        kdebug!(
                            "failed to create mmio 4k vma. pid = {:?}",
                            current_pcb().pid
                        );
                        unsafe {
                            vm_area_del(*vma);
                            vm_area_free(*vma);
                        }
                        return Err(SystemError::from_posix_errno(retval).unwrap());
                    }
                    loop_i += PAGE_4K_SIZE as u64;
                }
            }
            Err(_) => {
                kdebug!("failed to create mmio vma.pid = {:?}", current_pcb().pid);
                return Err(SystemError::ENOMEM);
            }
        }
        return Ok(retval);
    }

    /// @brief 取消mmio的映射并将地址空间归还到buddy中
    ///
    /// @param vaddr 起始的虚拟地址
    ///
    /// @param length 要归还的地址空间的长度
    ///
    /// @return Ok(i32) 成功返回0
    ///
    /// @return Err(SystemError) 失败返回错误码
    pub fn release_mmio(&self, vaddr: u64, length: u64) -> Result<i32, SystemError> {
        //先将要释放的空间取消映射
        unsafe {
            mm_unmap(&mut initial_mm, vaddr, length, false);
        }
        let mut loop_i: u64 = 0;
        loop {
            if loop_i >= length {
                break;
            }
            // 获取要释放的vma的结构体
            let vma: *mut vm_area_struct = unsafe { vma_find(&mut initial_mm, vaddr + loop_i) };
            if vma == null_mut() {
                kdebug!(
                    "mmio_release failed: vma not found. At address: {:?}, pid = {:?}",
                    vaddr + loop_i,
                    current_pcb().pid
                );
                return Err(SystemError::EINVAL);
            }
            // 检查vma起始地址是否正确
            if unsafe { (*vma).vm_start != (vaddr + loop_i) } {
                kdebug!(
                    "mmio_release failed: addr_start is not equal to current: {:?}. pid = {:?}",
                    vaddr + loop_i,
                    current_pcb().pid
                );
                return Err(SystemError::EINVAL);
            }
            // 将vma对应空间归还
            match MMIO_POOL.give_back_block(unsafe { (*vma).vm_start }, unsafe {
                31 - ((*vma).vm_end - (*vma).vm_start).leading_zeros()
            }) {
                Ok(_) => {
                    loop_i += unsafe { (*vma).vm_end - (*vma).vm_start };
                    unsafe {
                        vm_area_del(vma);
                        vm_area_free(vma);
                    }
                }
                Err(err) => {
                    // vma对应空间没有成功归还的话，就不删除vma
                    kdebug!(
                        "mmio_release give_back failed: pid = {:?}",
                        current_pcb().pid
                    );
                    return Err(err);
                }
            }
        }
        return Ok(0);
    }
}

/// @brief mmio伙伴系统内部的地址区域结构体
pub struct MmioBuddyAddrRegion {
    vaddr: u64,
}
impl MmioBuddyAddrRegion {
    pub fn new() -> Self {
        return MmioBuddyAddrRegion {
            ..Default::default()
        };
    }
}
impl Default for MmioBuddyAddrRegion {
    fn default() -> Self {
        MmioBuddyAddrRegion {
            vaddr: Default::default(),
        }
    }
}

/// @brief 空闲页数组结构体
pub struct MmioFreeRegionList {
    /// 存储mmio_buddy的地址链表
    list: LinkedList<Box<MmioBuddyAddrRegion>>,
    /// 空闲块的数量
    num_free: i64,
}
impl MmioFreeRegionList {
    #[allow(dead_code)]
    fn new() -> Self {
        return MmioFreeRegionList {
            ..Default::default()
        };
    }
}
impl Default for MmioFreeRegionList {
    fn default() -> Self {
        MmioFreeRegionList {
            list: Default::default(),
            num_free: 0,
        }
    }
}

/// @brief 初始化mmio的伙伴系统
#[no_mangle]
pub extern "C" fn __mmio_buddy_init() {
    // 创建一堆1GB的地址块
    let cnt_1g_blocks: u32 = ((MMIO_TOP - MMIO_BASE) / PAGE_1G_SIZE as i64) as u32;
    let mut vaddr_base: u64 = MMIO_BASE as u64;
    for _ in 0..cnt_1g_blocks {
        match MMIO_POOL.give_back_block(vaddr_base, PAGE_1G_SHIFT) {
            Ok(_) => {
                vaddr_base += PAGE_1G_SIZE as u64;
            }
            Err(_) => {
                kerror!("__mmio_buddy_init failed");
                return;
            }
        }
    }
}

/// @brief 将内存对象大小的幂转换成内存池中的数组的下标
///
/// @param exp内存大小
///
/// @return 内存池数组下标
#[inline(always)]
fn exp2index(exp: u32) -> usize {
    return (exp - 12) as usize;
}

/// @brief 创建一块mmio区域，并将vma绑定到initial_mm
///
/// @param size mmio区域的大小（字节）
///
/// @param vm_flags 要把vma设置成的标志
///
/// @param res_vaddr 返回值-分配得到的虚拟地址
///
/// @param res_length 返回值-分配的虚拟地址空间长度
///
/// @return int 错误码
#[no_mangle]
pub extern "C" fn mmio_create(
    size: u32,
    vm_flags: vm_flags_t,
    res_vaddr: *mut u64,
    res_length: *mut u64,
) -> i32 {
    if let Err(err) = MMIO_POOL.create_mmio(size, vm_flags, res_vaddr, res_length) {
        return err.to_posix_errno();
    } else {
        return 0;
    }
}

/// @brief 取消mmio的映射并将地址空间归还到buddy中
///
/// @param vaddr 起始的虚拟地址
///
/// @param length 要归还的地址空间的长度
///
/// @return Ok(i32) 成功返回0
///
/// @return Err(i32) 失败返回错误码
#[no_mangle]
pub extern "C" fn mmio_release(vaddr: u64, length: u64) -> i32 {
    if let Err(err) = MMIO_POOL.release_mmio(vaddr, length) {
        return err.to_posix_errno();
    } else {
        return 0;
    }
}