vm_memory/

guest_memory.rs

// Copyright (C) 2019 Alibaba Cloud Computing. All rights reserved.
//
// Portions Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
//
// 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.
//
// SPDX-License-Identifier: Apache-2.0 OR BSD-3-Clause

//! Traits to track and access the physical memory of the guest.
//!
//! To make the abstraction as generic as possible, all the core traits declared here only define
//! methods to access guest's memory, and never define methods to manage (create, delete, insert,
//! remove etc) guest's memory. This way, the guest memory consumers (virtio device drivers,
//! vhost drivers and boot loaders etc) may be decoupled from the guest memory provider (typically
//! a hypervisor).
//!
//! Traits and Structs
//! - [`GuestAddress`](struct.GuestAddress.html): represents a guest physical address (GPA).
//! - [`MemoryRegionAddress`](struct.MemoryRegionAddress.html): represents an offset inside a
//! region.
//! - [`GuestMemoryRegion`](trait.GuestMemoryRegion.html): represent a continuous region of guest's
//! physical memory.
//! - [`GuestMemory`](trait.GuestMemory.html): represent a collection of `GuestMemoryRegion`
//! objects.
//! The main responsibilities of the `GuestMemory` trait are:
//!     - hide the detail of accessing guest's physical address.
//!     - map a request address to a `GuestMemoryRegion` object and relay the request to it.
//!     - handle cases where an access request spanning two or more `GuestMemoryRegion` objects.
//!
//! Whenever a collection of `GuestMemoryRegion` objects is mutable,
//! [`GuestAddressSpace`](trait.GuestAddressSpace.html) should be implemented
//! for clients to obtain a [`GuestMemory`] reference or smart pointer.
//!
//! The `GuestMemoryRegion` trait has an associated `B: Bitmap` type which is used to handle
//! dirty bitmap tracking. Backends are free to define the granularity (or whether tracking is
//! actually performed at all). Those that do implement tracking functionality are expected to
//! ensure the correctness of the underlying `Bytes` implementation. The user has to explicitly
//! record (using the handle returned by `GuestRegionMmap::bitmap`) write accesses performed
//! via pointers, references, or slices returned by methods of `GuestMemory`,`GuestMemoryRegion`,
//! `VolatileSlice`, `VolatileRef`, or `VolatileArrayRef`.

use std::convert::From;
use std::fmt::{self, Display};
use std::fs::File;
use std::io::{self, Read, Write};
use std::ops::{BitAnd, BitOr, Deref};
use std::rc::Rc;
use std::sync::atomic::Ordering;
use std::sync::Arc;

use crate::address::{Address, AddressValue};
use crate::bitmap::{Bitmap, BS, MS};
use crate::bytes::{AtomicAccess, Bytes};
use crate::volatile_memory::{self, VolatileSlice};

static MAX_ACCESS_CHUNK: usize = 4096;

/// Errors associated with handling guest memory accesses.
#[allow(missing_docs)]
#[derive(Debug)]
pub enum Error {
    /// Failure in finding a guest address in any memory regions mapped by this guest.
    InvalidGuestAddress(GuestAddress),
    /// Couldn't read/write from the given source.
    IOError(io::Error),
    /// Incomplete read or write.
    PartialBuffer { expected: usize, completed: usize },
    /// Requested backend address is out of range.
    InvalidBackendAddress,
    /// Host virtual address not available.
    HostAddressNotAvailable,
}

impl From<volatile_memory::Error> for Error {
    fn from(e: volatile_memory::Error) -> Self {
        match e {
            volatile_memory::Error::OutOfBounds { .. } => Error::InvalidBackendAddress,
            volatile_memory::Error::Overflow { .. } => Error::InvalidBackendAddress,
            volatile_memory::Error::TooBig { .. } => Error::InvalidBackendAddress,
            volatile_memory::Error::Misaligned { .. } => Error::InvalidBackendAddress,
            volatile_memory::Error::IOError(e) => Error::IOError(e),
            volatile_memory::Error::PartialBuffer {
                expected,
                completed,
            } => Error::PartialBuffer {
                expected,
                completed,
            },
        }
    }
}

/// Result of guest memory operations.
pub type Result<T> = std::result::Result<T, Error>;

impl std::error::Error for Error {}

impl Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Guest memory error: ")?;
        match self {
            Error::InvalidGuestAddress(addr) => {
                write!(f, "invalid guest address {}", addr.raw_value())
            }
            Error::IOError(error) => write!(f, "{}", error),
            Error::PartialBuffer {
                expected,
                completed,
            } => write!(
                f,
                "only used {} bytes in {} long buffer",
                completed, expected,
            ),
            Error::InvalidBackendAddress => write!(f, "invalid backend address"),
            Error::HostAddressNotAvailable => write!(f, "host virtual address not available"),
        }
    }
}

/// Represents a guest physical address (GPA).
///
/// # Notes:
/// On ARM64, a 32-bit hypervisor may be used to support a 64-bit guest. For simplicity,
/// `u64` is used to store the the raw value no matter if the guest a 32-bit or 64-bit virtual
/// machine.
#[derive(Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd)]
pub struct GuestAddress(pub u64);
impl_address_ops!(GuestAddress, u64);

/// Represents an offset inside a region.
#[derive(Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd)]
pub struct MemoryRegionAddress(pub u64);
impl_address_ops!(MemoryRegionAddress, u64);

/// Type of the raw value stored in a `GuestAddress` object.
pub type GuestUsize = <GuestAddress as AddressValue>::V;

/// Represents the start point within a `File` that backs a `GuestMemoryRegion`.
#[derive(Clone, Debug)]
pub struct FileOffset {
    file: Arc<File>,
    start: u64,
}

impl FileOffset {
    /// Creates a new `FileOffset` object.
    pub fn new(file: File, start: u64) -> Self {
        FileOffset::from_arc(Arc::new(file), start)
    }

    /// Creates a new `FileOffset` object based on an exiting `Arc<File>`.
    pub fn from_arc(file: Arc<File>, start: u64) -> Self {
        FileOffset { file, start }
    }

    /// Returns a reference to the inner `File` object.
    pub fn file(&self) -> &File {
        self.file.as_ref()
    }

    /// Return a reference to the inner `Arc<File>` object.
    pub fn arc(&self) -> &Arc<File> {
        &self.file
    }

    /// Returns the start offset within the file.
    pub fn start(&self) -> u64 {
        self.start
    }
}

/// Represents a continuous region of guest physical memory.
#[allow(clippy::len_without_is_empty)]
pub trait GuestMemoryRegion: Bytes<MemoryRegionAddress, E = Error> {
    /// Type used for dirty memory tracking.
    type B: Bitmap;

    /// Returns the size of the region.
    fn len(&self) -> GuestUsize;

    /// Returns the minimum (inclusive) address managed by the region.
    fn start_addr(&self) -> GuestAddress;

    /// Returns the maximum (inclusive) address managed by the region.
    fn last_addr(&self) -> GuestAddress {
        // unchecked_add is safe as the region bounds were checked when it was created.
        self.start_addr().unchecked_add(self.len() - 1)
    }

    /// Borrow the associated `Bitmap` object.
    fn bitmap(&self) -> &Self::B;

    /// Returns the given address if it is within this region.
    fn check_address(&self, addr: MemoryRegionAddress) -> Option<MemoryRegionAddress> {
        if self.address_in_range(addr) {
            Some(addr)
        } else {
            None
        }
    }

    /// Returns `true` if the given address is within this region.
    fn address_in_range(&self, addr: MemoryRegionAddress) -> bool {
        addr.raw_value() < self.len()
    }

    /// Returns the address plus the offset if it is in this region.
    fn checked_offset(
        &self,
        base: MemoryRegionAddress,
        offset: usize,
    ) -> Option<MemoryRegionAddress> {
        base.checked_add(offset as u64)
            .and_then(|addr| self.check_address(addr))
    }

    /// Tries to convert an absolute address to a relative address within this region.
    ///
    /// Returns `None` if `addr` is out of the bounds of this region.
    fn to_region_addr(&self, addr: GuestAddress) -> Option<MemoryRegionAddress> {
        addr.checked_offset_from(self.start_addr())
            .and_then(|offset| self.check_address(MemoryRegionAddress(offset)))
    }

    /// Returns the host virtual address corresponding to the region address.
    ///
    /// Some [`GuestMemory`](trait.GuestMemory.html) implementations, like `GuestMemoryMmap`,
    /// have the capability to mmap guest address range into host virtual address space for
    /// direct access, so the corresponding host virtual address may be passed to other subsystems.
    ///
    /// # Note
    /// The underlying guest memory is not protected from memory aliasing, which breaks the
    /// Rust memory safety model. It's the caller's responsibility to ensure that there's no
    /// concurrent accesses to the underlying guest memory.
    fn get_host_address(&self, _addr: MemoryRegionAddress) -> Result<*mut u8> {
        Err(Error::HostAddressNotAvailable)
    }

    /// Returns information regarding the file and offset backing this memory region.
    fn file_offset(&self) -> Option<&FileOffset> {
        None
    }

    /// Returns a slice corresponding to the data in the region.
    ///
    /// Returns `None` if the region does not support slice-based access.
    ///
    /// # Safety
    ///
    /// Unsafe because of possible aliasing.
    unsafe fn as_slice(&self) -> Option<&[u8]> {
        None
    }

    /// Returns a mutable slice corresponding to the data in the region.
    ///
    /// Returns `None` if the region does not support slice-based access.
    ///
    /// # Safety
    ///
    /// Unsafe because of possible aliasing. Mutable accesses performed through the
    /// returned slice are not visible to the dirty bitmap tracking functionality of
    /// the region, and must be manually recorded using the associated bitmap object.
    unsafe fn as_mut_slice(&self) -> Option<&mut [u8]> {
        None
    }

    /// Returns a [`VolatileSlice`](struct.VolatileSlice.html) of `count` bytes starting at
    /// `offset`.
    #[allow(unused_variables)]
    fn get_slice(
        &self,
        offset: MemoryRegionAddress,
        count: usize,
    ) -> Result<VolatileSlice<BS<Self::B>>> {
        Err(Error::HostAddressNotAvailable)
    }

    /// Gets a slice of memory for the entire region that supports volatile access.
    ///
    /// # Examples (uses the `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// # use vm_memory::{GuestAddress, MmapRegion, GuestRegionMmap, GuestMemoryRegion};
    /// # use vm_memory::volatile_memory::{VolatileMemory, VolatileSlice, VolatileRef};
    /// #
    /// let region = MmapRegion::<()>::new(0x400).expect("Could not create mmap region");
    /// let region =
    ///     GuestRegionMmap::new(region, GuestAddress(0x0)).expect("Could not create guest memory");
    /// let slice = region
    ///     .as_volatile_slice()
    ///     .expect("Could not get volatile slice");
    ///
    /// let v = 42u32;
    /// let r = slice
    ///     .get_ref::<u32>(0x200)
    ///     .expect("Could not get reference");
    /// r.store(v);
    /// assert_eq!(r.load(), v);
    /// # }
    /// ```
    fn as_volatile_slice(&self) -> Result<VolatileSlice<BS<Self::B>>> {
        self.get_slice(MemoryRegionAddress(0), self.len() as usize)
    }

    /// Show if the region is based on the `HugeTLBFS`.
    /// Returns Some(true) if the region is backed by hugetlbfs.
    /// None represents that no information is available.
    ///
    /// # Examples (uses the `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// #   use vm_memory::{GuestAddress, GuestMemory, GuestMemoryMmap, GuestRegionMmap};
    /// let addr = GuestAddress(0x1000);
    /// let mem = GuestMemoryMmap::<()>::from_ranges(&[(addr, 0x1000)]).unwrap();
    /// let r = mem.find_region(addr).unwrap();
    /// assert_eq!(r.is_hugetlbfs(), None);
    /// # }
    /// ```
    #[cfg(target_os = "linux")]
    fn is_hugetlbfs(&self) -> Option<bool> {
        None
    }
}

/// `GuestAddressSpace` provides a way to retrieve a `GuestMemory` object.
/// The vm-memory crate already provides trivial implementation for
/// references to `GuestMemory` or reference-counted `GuestMemory` objects,
/// but the trait can also be implemented by any other struct in order
/// to provide temporary access to a snapshot of the memory map.
///
/// In order to support generic mutable memory maps, devices (or other things
/// that access memory) should store the memory as a `GuestAddressSpace<M>`.
/// This example shows that references can also be used as the `GuestAddressSpace`
/// implementation, providing a zero-cost abstraction whenever immutable memory
/// maps are sufficient.
///
/// # Examples (uses the `backend-mmap` and `backend-atomic` features)
///
/// ```
/// # #[cfg(feature = "backend-mmap")]
/// # {
/// # use std::sync::Arc;
/// # use vm_memory::{GuestAddress, GuestAddressSpace, GuestMemory, GuestMemoryMmap};
/// #
/// pub struct VirtioDevice<AS: GuestAddressSpace> {
///     mem: Option<AS>,
/// }
///
/// impl<AS: GuestAddressSpace> VirtioDevice<AS> {
///     fn new() -> Self {
///         VirtioDevice { mem: None }
///     }
///     fn activate(&mut self, mem: AS) {
///         self.mem = Some(mem)
///     }
/// }
///
/// fn get_mmap() -> GuestMemoryMmap<()> {
///     let start_addr = GuestAddress(0x1000);
///     GuestMemoryMmap::from_ranges(&vec![(start_addr, 0x400)])
///         .expect("Could not create guest memory")
/// }
///
/// // Using `VirtioDevice` with an immutable GuestMemoryMmap:
/// let mut for_immutable_mmap = VirtioDevice::<&GuestMemoryMmap<()>>::new();
/// let mmap = get_mmap();
/// for_immutable_mmap.activate(&mmap);
/// let mut another = VirtioDevice::<&GuestMemoryMmap<()>>::new();
/// another.activate(&mmap);
///
/// # #[cfg(feature = "backend-atomic")]
/// # {
/// # use vm_memory::GuestMemoryAtomic;
/// // Using `VirtioDevice` with a mutable GuestMemoryMmap:
/// let mut for_mutable_mmap = VirtioDevice::<GuestMemoryAtomic<GuestMemoryMmap<()>>>::new();
/// let atomic = GuestMemoryAtomic::new(get_mmap());
/// for_mutable_mmap.activate(atomic.clone());
/// let mut another = VirtioDevice::<GuestMemoryAtomic<GuestMemoryMmap<()>>>::new();
/// another.activate(atomic.clone());
///
/// // atomic can be modified here...
/// # }
/// # }
/// ```
pub trait GuestAddressSpace {
    /// The type that will be used to access guest memory.
    type M: GuestMemory;

    /// A type that provides access to the memory.
    type T: Clone + Deref<Target = Self::M>;

    /// Return an object (e.g. a reference or guard) that can be used
    /// to access memory through this address space.  The object provides
    /// a consistent snapshot of the memory map.
    fn memory(&self) -> Self::T;
}

impl<M: GuestMemory> GuestAddressSpace for &M {
    type M = M;
    type T = Self;

    fn memory(&self) -> Self {
        self
    }
}

impl<M: GuestMemory> GuestAddressSpace for Rc<M> {
    type M = M;
    type T = Self;

    fn memory(&self) -> Self {
        self.clone()
    }
}

impl<M: GuestMemory> GuestAddressSpace for Arc<M> {
    type M = M;
    type T = Self;

    fn memory(&self) -> Self {
        self.clone()
    }
}

/// Lifetime generic associated iterators. The actual iterator type is defined through associated
/// item `Iter`, for example:
///
/// ```
/// # use std::marker::PhantomData;
/// # use vm_memory::guest_memory::GuestMemoryIterator;
/// #
/// // Declare the relevant Region and Memory types
/// struct MyGuestRegion {/* fields omitted */}
/// struct MyGuestMemory {/* fields omitted */}
///
/// // Make an Iterator type to iterate over the Regions
/// # /*
/// struct MyGuestMemoryIter<'a> {/* fields omitted */}
/// # */
/// # struct MyGuestMemoryIter<'a> {
/// #   _marker: PhantomData<&'a MyGuestRegion>,
/// # }
/// impl<'a> Iterator for MyGuestMemoryIter<'a> {
///     type Item = &'a MyGuestRegion;
///     fn next(&mut self) -> Option<&'a MyGuestRegion> {
///         // ...
/// #       None
///     }
/// }
///
/// // Associate the Iter type with the Memory type
/// impl<'a> GuestMemoryIterator<'a, MyGuestRegion> for MyGuestMemory {
///     type Iter = MyGuestMemoryIter<'a>;
/// }
/// ```
pub trait GuestMemoryIterator<'a, R: 'a> {
    /// Type of the `iter` method's return value.
    type Iter: Iterator<Item = &'a R>;
}

/// `GuestMemory` represents a container for an *immutable* collection of
/// `GuestMemoryRegion` objects.  `GuestMemory` provides the `Bytes<GuestAddress>`
/// trait to hide the details of accessing guest memory by physical address.
/// Interior mutability is not allowed for implementations of `GuestMemory` so
/// that they always provide a consistent view of the memory map.
///
/// The task of the `GuestMemory` trait are:
/// - map a request address to a `GuestMemoryRegion` object and relay the request to it.
/// - handle cases where an access request spanning two or more `GuestMemoryRegion` objects.
pub trait GuestMemory {
    /// Type of objects hosted by the address space.
    type R: GuestMemoryRegion;

    /// Lifetime generic associated iterators. Usually this is just `Self`.
    type I: for<'a> GuestMemoryIterator<'a, Self::R>;

    /// Returns the number of regions in the collection.
    fn num_regions(&self) -> usize;

    /// Returns the region containing the specified address or `None`.
    fn find_region(&self, addr: GuestAddress) -> Option<&Self::R>;

    /// Perform the specified action on each region.
    ///
    /// It only walks children of current region and does not step into sub regions.
    #[deprecated(since = "0.6.0", note = "Use `.iter()` instead")]
    fn with_regions<F, E>(&self, cb: F) -> std::result::Result<(), E>
    where
        F: Fn(usize, &Self::R) -> std::result::Result<(), E>,
    {
        for (index, region) in self.iter().enumerate() {
            cb(index, region)?;
        }
        Ok(())
    }

    /// Perform the specified action on each region mutably.
    ///
    /// It only walks children of current region and does not step into sub regions.
    #[deprecated(since = "0.6.0", note = "Use `.iter()` instead")]
    fn with_regions_mut<F, E>(&self, mut cb: F) -> std::result::Result<(), E>
    where
        F: FnMut(usize, &Self::R) -> std::result::Result<(), E>,
    {
        for (index, region) in self.iter().enumerate() {
            cb(index, region)?;
        }
        Ok(())
    }

    /// Gets an iterator over the entries in the collection.
    ///
    /// # Examples
    ///
    /// * Compute the total size of all memory mappings in KB by iterating over the memory regions
    ///   and dividing their sizes to 1024, then summing up the values in an accumulator. (uses the
    ///  `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// # use vm_memory::{GuestAddress, GuestMemory, GuestMemoryRegion, GuestMemoryMmap};
    /// #
    /// let start_addr1 = GuestAddress(0x0);
    /// let start_addr2 = GuestAddress(0x400);
    /// let gm = GuestMemoryMmap::<()>::from_ranges(&vec![(start_addr1, 1024), (start_addr2, 2048)])
    ///     .expect("Could not create guest memory");
    ///
    /// let total_size = gm
    ///     .iter()
    ///     .map(|region| region.len() / 1024)
    ///     .fold(0, |acc, size| acc + size);
    /// assert_eq!(3, total_size)
    /// # }
    /// ```
    fn iter(&self) -> <Self::I as GuestMemoryIterator<Self::R>>::Iter;

    /// Applies two functions, specified as callbacks, on the inner memory regions.
    ///
    /// # Arguments
    /// * `init` - Starting value of the accumulator for the `foldf` function.
    /// * `mapf` - "Map" function, applied to all the inner memory regions. It returns an array of
    ///            the same size as the memory regions array, containing the function's results
    ///            for each region.
    /// * `foldf` - "Fold" function, applied to the array returned by `mapf`. It acts as an
    ///             operator, applying itself to the `init` value and to each subsequent elemnent
    ///             in the array returned by `mapf`.
    ///
    /// # Examples
    ///
    /// * Compute the total size of all memory mappings in KB by iterating over the memory regions
    ///   and dividing their sizes to 1024, then summing up the values in an accumulator. (uses the
    ///  `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// # use vm_memory::{GuestAddress, GuestMemory, GuestMemoryRegion, GuestMemoryMmap};
    /// #
    /// let start_addr1 = GuestAddress(0x0);
    /// let start_addr2 = GuestAddress(0x400);
    /// let gm = GuestMemoryMmap::<()>::from_ranges(&vec![(start_addr1, 1024), (start_addr2, 2048)])
    ///     .expect("Could not create guest memory");
    ///
    /// let total_size = gm.map_and_fold(0, |(_, region)| region.len() / 1024, |acc, size| acc + size);
    /// assert_eq!(3, total_size)
    /// # }
    /// ```
    #[deprecated(since = "0.6.0", note = "Use `.iter()` instead")]
    fn map_and_fold<F, G, T>(&self, init: T, mapf: F, foldf: G) -> T
    where
        F: Fn((usize, &Self::R)) -> T,
        G: Fn(T, T) -> T,
    {
        self.iter().enumerate().map(mapf).fold(init, foldf)
    }

    /// Returns the maximum (inclusive) address managed by the
    /// [`GuestMemory`](trait.GuestMemory.html).
    ///
    /// # Examples (uses the `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// # use vm_memory::{Address, GuestAddress, GuestMemory, GuestMemoryMmap};
    /// #
    /// let start_addr = GuestAddress(0x1000);
    /// let mut gm = GuestMemoryMmap::<()>::from_ranges(&vec![(start_addr, 0x400)])
    ///     .expect("Could not create guest memory");
    ///
    /// assert_eq!(start_addr.checked_add(0x3ff), Some(gm.last_addr()));
    /// # }
    /// ```
    fn last_addr(&self) -> GuestAddress {
        self.iter()
            .map(GuestMemoryRegion::last_addr)
            .fold(GuestAddress(0), std::cmp::max)
    }

    /// Tries to convert an absolute address to a relative address within the corresponding region.
    ///
    /// Returns `None` if `addr` isn't present within the memory of the guest.
    fn to_region_addr(&self, addr: GuestAddress) -> Option<(&Self::R, MemoryRegionAddress)> {
        self.find_region(addr)
            .map(|r| (r, r.to_region_addr(addr).unwrap()))
    }

    /// Returns `true` if the given address is present within the memory of the guest.
    fn address_in_range(&self, addr: GuestAddress) -> bool {
        self.find_region(addr).is_some()
    }

    /// Returns the given address if it is present within the memory of the guest.
    fn check_address(&self, addr: GuestAddress) -> Option<GuestAddress> {
        self.find_region(addr).map(|_| addr)
    }

    /// Check whether the range [base, base + len) is valid.
    fn check_range(&self, base: GuestAddress, len: usize) -> bool {
        match self.try_access(len, base, |_, count, _, _| -> Result<usize> { Ok(count) }) {
            Ok(count) => count == len,
            _ => false,
        }
    }

    /// Returns the address plus the offset if it is present within the memory of the guest.
    fn checked_offset(&self, base: GuestAddress, offset: usize) -> Option<GuestAddress> {
        base.checked_add(offset as u64)
            .and_then(|addr| self.check_address(addr))
    }

    /// Invokes callback `f` to handle data in the address range `[addr, addr + count)`.
    ///
    /// The address range `[addr, addr + count)` may span more than one
    /// [`GuestMemoryRegion`](trait.GuestMemoryRegion.html) object, or even have holes in it.
    /// So [`try_access()`](trait.GuestMemory.html#method.try_access) invokes the callback 'f'
    /// for each [`GuestMemoryRegion`](trait.GuestMemoryRegion.html) object involved and returns:
    /// - the error code returned by the callback 'f'
    /// - the size of the already handled data when encountering the first hole
    /// - the size of the already handled data when the whole range has been handled
    fn try_access<F>(&self, count: usize, addr: GuestAddress, mut f: F) -> Result<usize>
    where
        F: FnMut(usize, usize, MemoryRegionAddress, &Self::R) -> Result<usize>,
    {
        let mut cur = addr;
        let mut total = 0;
        while let Some(region) = self.find_region(cur) {
            let start = region.to_region_addr(cur).unwrap();
            let cap = region.len() - start.raw_value();
            let len = std::cmp::min(cap, (count - total) as GuestUsize);
            match f(total, len as usize, start, region) {
                // no more data
                Ok(0) => return Ok(total),
                // made some progress
                Ok(len) => {
                    total += len;
                    if total == count {
                        break;
                    }
                    cur = match cur.overflowing_add(len as GuestUsize) {
                        (GuestAddress(0), _) => GuestAddress(0),
                        (result, false) => result,
                        (_, true) => panic!("guest address overflow"),
                    }
                }
                // error happened
                e => return e,
            }
        }
        if total == 0 {
            Err(Error::InvalidGuestAddress(addr))
        } else {
            Ok(total)
        }
    }

    /// Get the host virtual address corresponding to the guest address.
    ///
    /// Some [`GuestMemory`](trait.GuestMemory.html) implementations, like `GuestMemoryMmap`,
    /// have the capability to mmap the guest address range into virtual address space of the host
    /// for direct access, so the corresponding host virtual address may be passed to other
    /// subsystems.
    ///
    /// # Note
    /// The underlying guest memory is not protected from memory aliasing, which breaks the
    /// Rust memory safety model. It's the caller's responsibility to ensure that there's no
    /// concurrent accesses to the underlying guest memory.
    ///
    /// # Arguments
    /// * `addr` - Guest address to convert.
    ///
    /// # Examples (uses the `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// # use vm_memory::{GuestAddress, GuestMemory, GuestMemoryMmap};
    /// #
    /// # let start_addr = GuestAddress(0x1000);
    /// # let mut gm = GuestMemoryMmap::<()>::from_ranges(&vec![(start_addr, 0x500)])
    /// #    .expect("Could not create guest memory");
    /// #
    /// let addr = gm
    ///     .get_host_address(GuestAddress(0x1200))
    ///     .expect("Could not get host address");
    /// println!("Host address is {:p}", addr);
    /// # }
    /// ```
    fn get_host_address(&self, addr: GuestAddress) -> Result<*mut u8> {
        self.to_region_addr(addr)
            .ok_or(Error::InvalidGuestAddress(addr))
            .and_then(|(r, addr)| r.get_host_address(addr))
    }

    /// Returns a [`VolatileSlice`](struct.VolatileSlice.html) of `count` bytes starting at
    /// `addr`.
    fn get_slice(&self, addr: GuestAddress, count: usize) -> Result<VolatileSlice<MS<Self>>> {
        self.to_region_addr(addr)
            .ok_or(Error::InvalidGuestAddress(addr))
            .and_then(|(r, addr)| r.get_slice(addr, count))
    }
}

impl<T: GuestMemory + ?Sized> Bytes<GuestAddress> for T {
    type E = Error;

    fn write(&self, buf: &[u8], addr: GuestAddress) -> Result<usize> {
        self.try_access(
            buf.len(),
            addr,
            |offset, _count, caddr, region| -> Result<usize> {
                region.write(&buf[offset as usize..], caddr)
            },
        )
    }

    fn read(&self, buf: &mut [u8], addr: GuestAddress) -> Result<usize> {
        self.try_access(
            buf.len(),
            addr,
            |offset, _count, caddr, region| -> Result<usize> {
                region.read(&mut buf[offset as usize..], caddr)
            },
        )
    }

    /// # Examples
    ///
    /// * Write a slice at guestaddress 0x1000. (uses the `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// # use vm_memory::{Bytes, GuestAddress, mmap::GuestMemoryMmap};
    /// #
    /// # let start_addr = GuestAddress(0x1000);
    /// # let mut gm = GuestMemoryMmap::<()>::from_ranges(&vec![(start_addr, 0x400)])
    /// #    .expect("Could not create guest memory");
    /// #
    /// gm.write_slice(&[1, 2, 3, 4, 5], start_addr)
    ///     .expect("Could not write slice to guest memory");
    /// # }
    /// ```
    fn write_slice(&self, buf: &[u8], addr: GuestAddress) -> Result<()> {
        let res = self.write(buf, addr)?;
        if res != buf.len() {
            return Err(Error::PartialBuffer {
                expected: buf.len(),
                completed: res,
            });
        }
        Ok(())
    }

    /// # Examples
    ///
    /// * Read a slice of length 16 at guestaddress 0x1000. (uses the `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// # use vm_memory::{Bytes, GuestAddress, mmap::GuestMemoryMmap};
    /// #
    /// let start_addr = GuestAddress(0x1000);
    /// let mut gm = GuestMemoryMmap::<()>::from_ranges(&vec![(start_addr, 0x400)])
    ///     .expect("Could not create guest memory");
    /// let buf = &mut [0u8; 16];
    ///
    /// gm.read_slice(buf, start_addr)
    ///     .expect("Could not read slice from guest memory");
    /// # }
    /// ```
    fn read_slice(&self, buf: &mut [u8], addr: GuestAddress) -> Result<()> {
        let res = self.read(buf, addr)?;
        if res != buf.len() {
            return Err(Error::PartialBuffer {
                expected: buf.len(),
                completed: res,
            });
        }
        Ok(())
    }

    /// # Examples
    ///
    /// * Read bytes from /dev/urandom (uses the `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// # use vm_memory::{Address, Bytes, GuestAddress, GuestMemoryMmap};
    /// # use std::fs::File;
    /// # use std::path::Path;
    /// #
    /// # let start_addr = GuestAddress(0x1000);
    /// # let gm = GuestMemoryMmap::<()>::from_ranges(&vec![(start_addr, 0x400)])
    /// #    .expect("Could not create guest memory");
    /// # let addr = GuestAddress(0x1010);
    /// # let mut file = if cfg!(unix) {
    /// let mut file = File::open(Path::new("/dev/urandom")).expect("Could not open /dev/urandom");
    /// #   file
    /// # } else {
    /// #   File::open(Path::new("c:\\Windows\\system32\\ntoskrnl.exe"))
    /// #       .expect("Could not open c:\\Windows\\system32\\ntoskrnl.exe")
    /// # };
    ///
    /// gm.read_from(addr, &mut file, 128)
    ///     .expect("Could not read from /dev/urandom into guest memory");
    ///
    /// let read_addr = addr.checked_add(8).expect("Could not compute read address");
    /// let rand_val: u32 = gm
    ///     .read_obj(read_addr)
    ///     .expect("Could not read u32 val from /dev/urandom");
    /// # }
    /// ```
    fn read_from<F>(&self, addr: GuestAddress, src: &mut F, count: usize) -> Result<usize>
    where
        F: Read,
    {
        self.try_access(count, addr, |offset, len, caddr, region| -> Result<usize> {
            // Check if something bad happened before doing unsafe things.
            assert!(offset <= count);
            // SAFETY: Safe because we are checking the offset.
            if let Some(dst) = unsafe { region.as_mut_slice() } {
                // This is safe cause `start` and `len` are within the `region`, and we manually
                // record the dirty status of the written range below.
                let start = caddr.raw_value() as usize;
                let end = start + len;
                let bytes_read = loop {
                    match src.read(&mut dst[start..end]) {
                        Ok(n) => break n,
                        Err(ref e) if e.kind() == std::io::ErrorKind::Interrupted => continue,
                        Err(e) => return Err(Error::IOError(e)),
                    }
                };

                region.bitmap().mark_dirty(start, bytes_read);
                Ok(bytes_read)
            } else {
                let len = std::cmp::min(len, MAX_ACCESS_CHUNK);
                let mut buf = vec![0u8; len].into_boxed_slice();
                loop {
                    match src.read(&mut buf[..]) {
                        Ok(bytes_read) => {
                            // We don't need to update the dirty bitmap manually here because it's
                            // expected to be handled by the logic within the `Bytes`
                            // implementation for the region object.
                            let bytes_written = region.write(&buf[0..bytes_read], caddr)?;
                            assert_eq!(bytes_written, bytes_read);
                            break Ok(bytes_read);
                        }
                        Err(ref e) if e.kind() == std::io::ErrorKind::Interrupted => continue,
                        Err(e) => break Err(Error::IOError(e)),
                    }
                }
            }
        })
    }

    fn read_exact_from<F>(&self, addr: GuestAddress, src: &mut F, count: usize) -> Result<()>
    where
        F: Read,
    {
        let res = self.read_from(addr, src, count)?;
        if res != count {
            return Err(Error::PartialBuffer {
                expected: count,
                completed: res,
            });
        }
        Ok(())
    }

    /// # Examples
    ///
    /// * Write 128 bytes to /dev/null (uses the `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(not(unix))]
    /// # extern crate vmm_sys_util;
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// # use vm_memory::{Bytes, GuestAddress, GuestMemoryMmap};
    /// #
    /// # let start_addr = GuestAddress(0x1000);
    /// # let gm = GuestMemoryMmap::<()>::from_ranges(&vec![(start_addr, 1024)])
    /// #    .expect("Could not create guest memory");
    /// # let mut file = if cfg!(unix) {
    /// # use std::fs::OpenOptions;
    /// let mut file = OpenOptions::new()
    ///     .write(true)
    ///     .open("/dev/null")
    ///     .expect("Could not open /dev/null");
    /// #   file
    /// # } else {
    /// #   use vmm_sys_util::tempfile::TempFile;
    /// #   TempFile::new().unwrap().into_file()
    /// # };
    ///
    /// gm.write_to(start_addr, &mut file, 128)
    ///     .expect("Could not write 128 bytes to the provided address");
    /// # }
    /// ```
    fn write_to<F>(&self, addr: GuestAddress, dst: &mut F, count: usize) -> Result<usize>
    where
        F: Write,
    {
        self.try_access(count, addr, |offset, len, caddr, region| -> Result<usize> {
            // Check if something bad happened before doing unsafe things.
            assert!(offset <= count);
            // SAFETY: Safe because we are checking the offset is valid.
            if let Some(src) = unsafe { region.as_slice() } {
                // This is safe cause `start` and `len` are within the `region`.
                let start = caddr.raw_value() as usize;
                let end = start + len;
                loop {
                    // It is safe to read from volatile memory. Accessing the guest
                    // memory as a slice should be OK as long as nothing assumes another
                    // thread won't change what is loaded; however, we may want to introduce
                    // VolatileRead and VolatileWrite traits in the future.
                    match dst.write(&src[start..end]) {
                        Ok(n) => break Ok(n),
                        Err(ref e) if e.kind() == std::io::ErrorKind::Interrupted => continue,
                        Err(e) => break Err(Error::IOError(e)),
                    }
                }
            } else {
                let len = std::cmp::min(len, MAX_ACCESS_CHUNK);
                let mut buf = vec![0u8; len].into_boxed_slice();
                let bytes_read = region.read(&mut buf, caddr)?;
                assert_eq!(bytes_read, len);
                // For a non-RAM region, reading could have side effects, so we
                // must use write_all().
                dst.write_all(&buf).map_err(Error::IOError)?;
                Ok(len)
            }
        })
    }

    /// # Examples
    ///
    /// * Write 128 bytes to /dev/null (uses the `backend-mmap` feature)
    ///
    /// ```
    /// # #[cfg(not(unix))]
    /// # extern crate vmm_sys_util;
    /// # #[cfg(feature = "backend-mmap")]
    /// # {
    /// # use vm_memory::{Bytes, GuestAddress, GuestMemoryMmap};
    /// #
    /// # let start_addr = GuestAddress(0x1000);
    /// # let gm = GuestMemoryMmap::<()>::from_ranges(&vec![(start_addr, 1024)])
    /// #    .expect("Could not create guest memory");
    /// # let mut file = if cfg!(unix) {
    /// # use std::fs::OpenOptions;
    /// let mut file = OpenOptions::new()
    ///     .write(true)
    ///     .open("/dev/null")
    ///     .expect("Could not open /dev/null");
    /// #   file
    /// # } else {
    /// #   use vmm_sys_util::tempfile::TempFile;
    /// #   TempFile::new().unwrap().into_file()
    /// # };
    ///
    /// gm.write_all_to(start_addr, &mut file, 128)
    ///     .expect("Could not write 128 bytes to the provided address");
    /// # }
    /// ```
    fn write_all_to<F>(&self, addr: GuestAddress, dst: &mut F, count: usize) -> Result<()>
    where
        F: Write,
    {
        let res = self.write_to(addr, dst, count)?;
        if res != count {
            return Err(Error::PartialBuffer {
                expected: count,
                completed: res,
            });
        }
        Ok(())
    }

    fn store<O: AtomicAccess>(&self, val: O, addr: GuestAddress, order: Ordering) -> Result<()> {
        // `find_region` should really do what `to_region_addr` is doing right now, except
        // it should keep returning a `Result`.
        self.to_region_addr(addr)
            .ok_or(Error::InvalidGuestAddress(addr))
            .and_then(|(region, region_addr)| region.store(val, region_addr, order))
    }

    fn load<O: AtomicAccess>(&self, addr: GuestAddress, order: Ordering) -> Result<O> {
        self.to_region_addr(addr)
            .ok_or(Error::InvalidGuestAddress(addr))
            .and_then(|(region, region_addr)| region.load(region_addr, order))
    }
}

#[cfg(test)]
mod tests {
    #![allow(clippy::undocumented_unsafe_blocks)]
    use super::*;
    #[cfg(feature = "backend-mmap")]
    use crate::bytes::ByteValued;
    #[cfg(feature = "backend-mmap")]
    use crate::GuestAddress;
    #[cfg(feature = "backend-mmap")]
    use std::io::Cursor;
    #[cfg(feature = "backend-mmap")]
    use std::time::{Duration, Instant};

    use vmm_sys_util::tempfile::TempFile;

    #[cfg(feature = "backend-mmap")]
    type GuestMemoryMmap = crate::GuestMemoryMmap<()>;

    #[cfg(feature = "backend-mmap")]
    fn make_image(size: u8) -> Vec<u8> {
        let mut image: Vec<u8> = Vec::with_capacity(size as usize);
        for i in 0..size {
            image.push(i);
        }
        image
    }

    #[test]
    fn test_file_offset() {
        let file = TempFile::new().unwrap().into_file();
        let start = 1234;
        let file_offset = FileOffset::new(file, start);
        assert_eq!(file_offset.start(), start);
        assert_eq!(
            file_offset.file() as *const File,
            file_offset.arc().as_ref() as *const File
        );
    }

    #[cfg(feature = "backend-mmap")]
    #[test]
    fn checked_read_from() {
        let start_addr1 = GuestAddress(0x0);
        let start_addr2 = GuestAddress(0x40);
        let mem = GuestMemoryMmap::from_ranges(&[(start_addr1, 64), (start_addr2, 64)]).unwrap();
        let image = make_image(0x80);
        let offset = GuestAddress(0x30);
        let count: usize = 0x20;
        assert_eq!(
            0x20_usize,
            mem.read_from(offset, &mut Cursor::new(&image), count)
                .unwrap()
        );
    }

    // Runs the provided closure in a loop, until at least `duration` time units have elapsed.
    #[cfg(feature = "backend-mmap")]
    fn loop_timed<F>(duration: Duration, mut f: F)
    where
        F: FnMut(),
    {
        // We check the time every `CHECK_PERIOD` iterations.
        const CHECK_PERIOD: u64 = 1_000_000;
        let start_time = Instant::now();

        loop {
            for _ in 0..CHECK_PERIOD {
                f();
            }
            if start_time.elapsed() >= duration {
                break;
            }
        }
    }

    // Helper method for the following test. It spawns a writer and a reader thread, which
    // simultaneously try to access an object that is placed at the junction of two memory regions.
    // The part of the object that's continuously accessed is a member of type T. The writer
    // flips all the bits of the member with every write, while the reader checks that every byte
    // has the same value (and thus it did not do a non-atomic access). The test succeeds if
    // no mismatch is detected after performing accesses for a pre-determined amount of time.
    #[cfg(feature = "backend-mmap")]
    fn non_atomic_access_helper<T>()
    where
        T: ByteValued
            + std::fmt::Debug
            + From<u8>
            + Into<u128>
            + std::ops::Not<Output = T>
            + PartialEq,
    {
        use std::mem;
        use std::thread;

        // A dummy type that's always going to have the same alignment as the first member,
        // and then adds some bytes at the end.
        #[derive(Clone, Copy, Debug, Default, PartialEq)]
        struct Data<T> {
            val: T,
            some_bytes: [u8; 7],
        }

        // Some sanity checks.
        assert_eq!(mem::align_of::<T>(), mem::align_of::<Data<T>>());
        assert_eq!(mem::size_of::<T>(), mem::align_of::<T>());

        unsafe impl<T: ByteValued> ByteValued for Data<T> {}

        // Start of first guest memory region.
        let start = GuestAddress(0);
        let region_len = 1 << 12;

        // The address where we start writing/reading a Data<T> value.
        let data_start = GuestAddress((region_len - mem::size_of::<T>()) as u64);

        let mem = GuestMemoryMmap::from_ranges(&[
            (start, region_len),
            (start.unchecked_add(region_len as u64), region_len),
        ])
        .unwrap();

        // Need to clone this and move it into the new thread we create.
        let mem2 = mem.clone();
        // Just some bytes.
        let some_bytes = [1u8, 2, 4, 16, 32, 64, 128];

        let mut data = Data {
            val: T::from(0u8),
            some_bytes,
        };

        // Simple check that cross-region write/read is ok.
        mem.write_obj(data, data_start).unwrap();
        let read_data = mem.read_obj::<Data<T>>(data_start).unwrap();
        assert_eq!(read_data, data);

        let t = thread::spawn(move || {
            let mut count: u64 = 0;

            loop_timed(Duration::from_secs(3), || {
                let data = mem2.read_obj::<Data<T>>(data_start).unwrap();

                // Every time data is written to memory by the other thread, the value of
                // data.val alternates between 0 and T::MAX, so the inner bytes should always
                // have the same value. If they don't match, it means we read a partial value,
                // so the access was not atomic.
                let bytes = data.val.into().to_le_bytes();
                for i in 1..mem::size_of::<T>() {
                    if bytes[0] != bytes[i] {
                        panic!(
                            "val bytes don't match {:?} after {} iterations",
                            &bytes[..mem::size_of::<T>()],
                            count
                        );
                    }
                }
                count += 1;
            });
        });

        // Write the object while flipping the bits of data.val over and over again.
        loop_timed(Duration::from_secs(3), || {
            mem.write_obj(data, data_start).unwrap();
            data.val = !data.val;
        });

        t.join().unwrap()
    }

    #[cfg(feature = "backend-mmap")]
    #[test]
    fn test_non_atomic_access() {
        non_atomic_access_helper::<u16>()
    }

    #[cfg(feature = "backend-mmap")]
    #[test]
    fn test_zero_length_accesses() {
        #[derive(Default, Clone, Copy)]
        #[repr(C)]
        struct ZeroSizedStruct {
            dummy: [u32; 0],
        }

        unsafe impl ByteValued for ZeroSizedStruct {}

        let addr = GuestAddress(0x1000);
        let mem = GuestMemoryMmap::from_ranges(&[(addr, 0x1000)]).unwrap();
        let obj = ZeroSizedStruct::default();
        let mut image = make_image(0x80);

        assert_eq!(mem.write(&[], addr).unwrap(), 0);
        assert_eq!(mem.read(&mut [], addr).unwrap(), 0);

        assert!(mem.write_slice(&[], addr).is_ok());
        assert!(mem.read_slice(&mut [], addr).is_ok());

        assert!(mem.write_obj(obj, addr).is_ok());
        assert!(mem.read_obj::<ZeroSizedStruct>(addr).is_ok());

        assert_eq!(mem.read_from(addr, &mut Cursor::new(&image), 0).unwrap(), 0);

        assert!(mem
            .read_exact_from(addr, &mut Cursor::new(&image), 0)
            .is_ok());

        assert_eq!(
            mem.write_to(addr, &mut Cursor::new(&mut image), 0).unwrap(),
            0
        );

        assert!(mem
            .write_all_to(addr, &mut Cursor::new(&mut image), 0)
            .is_ok());
    }

    #[cfg(feature = "backend-mmap")]
    #[test]
    fn test_atomic_accesses() {
        let addr = GuestAddress(0x1000);
        let mem = GuestMemoryMmap::from_ranges(&[(addr, 0x1000)]).unwrap();
        let bad_addr = addr.unchecked_add(0x1000);

        crate::bytes::tests::check_atomic_accesses(mem, addr, bad_addr);
    }

    #[cfg(feature = "backend-mmap")]
    #[cfg(target_os = "linux")]
    #[test]
    fn test_guest_memory_mmap_is_hugetlbfs() {
        let addr = GuestAddress(0x1000);
        let mem = GuestMemoryMmap::from_ranges(&[(addr, 0x1000)]).unwrap();
        let r = mem.find_region(addr).unwrap();
        assert_eq!(r.is_hugetlbfs(), None);
    }
}