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DESIGN.md

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Design

Objectives

  • Provide a set of traits for accessing and configuring the physical memory of a virtual machine.
  • Provide a clean abstraction of the VM memory such that rust-vmm components can use it without depending on the implementation details specific to different VMMs.

API Principles

  • Define consumer side interfaces to access VM's physical memory.
  • Do not define provider side interfaces to supply VM physical memory.

The vm-memory crate focuses on defining consumer side interfaces to access the physical memory of the VM. It does not define how the underlying VM memory provider is implemented. Lightweight VMMs like CrosVM and Firecracker can make assumptions about the structure of VM's physical memory and implement a lightweight backend to access it. For VMMs like Qemu, a high performance and full functionality backend may be implemented with less assumptions.

Architecture

The vm-memory is derived from two upstream projects:

  • CrosVM commit 186eb8b0db644892e8ffba8344efe3492bb2b823
  • Firecracker commit 80128ea61b305a27df1f751d70415b04b503eae7

The high level abstraction of the VM memory has been heavily refactored to provide a VMM agnostic interface.

The vm-memory crate could be divided into four logic parts as:

Address Space Abstraction

The address space abstraction contains traits and implementations for working with addresses as follows:

  • AddressValue: stores the raw value of an address. Typically u32, u64 or usize are used to store the raw value. Pointers such as *u8, can not be used as an implementation of AddressValue because the Add and Sub traits are not implemented for that type.
  • Address: implementation of AddressValue.
  • Bytes: trait for volatile access to memory. The Bytes trait can be parameterized with types that represent addresses, in order to enforce that addresses are used with the right "kind" of volatile memory.
  • VolatileMemory: basic implementation of volatile access to memory. Implements Bytes<usize>.

To make the abstraction as generic as possible, all of above traits only define methods to access the address space, and they never define methods to manage (create, delete, insert, remove etc) address spaces. This way, the address space consumers may be decoupled from the address space provider (typically a VMM).

Specialization for Virtual Machine Physical Address Space

The generic address space crates are specialized to access the physical memory of the VM using the following traits:

  • GuestAddress: represents a guest physical address (GPA). On ARM64, a 32-bit VMM/hypervisor can be used to support a 64-bit VM. For simplicity, u64 is used to store the the raw value no matter if it is a 32-bit or a 64-bit virtual machine.
  • GuestMemoryRegion: represents a continuous region of the VM memory.
  • GuestMemory: represents a collection of GuestMemoryRegion objects. The main responsibilities of the GuestMemory trait are:
    • hide the detail of accessing physical addresses (for example complex hierarchical structures).
    • map an address request to a GuestMemoryRegion object and relay the request to it.
    • handle cases where an access request is spanning two or more GuestMemoryRegion objects.

The VM memory consumers should only rely on traits and structs defined here to access VM's physical memory and not on the implementation of the traits.

Backend Implementation Based on mmap

Provides an implementation of the GuestMemory trait by mmapping the VM's physical memory into the current process.

  • MmapRegion: implementation of mmap a continuous range of physical memory with methods for accessing the mapped memory.
  • GuestRegionMmap: implementation of GuestMemoryRegion providing a wrapper used to map VM's physical address into a (mmap_region, offset) tuple.
  • GuestMemoryMmap: implementation of GuestMemory that manages a collection of GuestRegionMmap objects for a VM.

One of the main responsibilities of GuestMemoryMmap is to handle the use cases where an access request crosses the memory region boundary. This scenario may be triggered when memory hotplug is supported. There is a trade-off between simplicity and code complexity:

  • The following pattern currently used in both CrosVM and Firecracker is simple, but fails when the request crosses region boundary.
let guest_memory_mmap: GuestMemoryMmap = ...
let addr: GuestAddress = ...
let buf = &mut [0u8; 5];
let result = guest_memory_mmap.find_region(addr).unwrap().write(buf, addr);
  • To support requests crossing region boundary, the following update is needed:
let guest_memory_mmap: GuestMemoryMmap = ...
let addr: GuestAddress = ...
let buf = &mut [0u8; 5];
let result = guest_memory_mmap.write(buf, addr);

Utilities and Helpers

The following utilities and helper traits/macros are imported from the crosvm project with minor changes:

  • ByteValued (originally DataInit): types which are safe to be initialized from raw data. A type T is ByteValued if and only if it can be initialized by reading its contents from a byte array. This is generally true for all plain-old-data structs. It is notably not true for any type that includes a reference.
  • {Le,Be}_{16,32,64}: explicit endian types useful for embedding in structs or reinterpreting data.

Relationships between Traits, Structs and Types

Traits:

  • Address inherits AddressValue
  • GuestMemoryRegion inherits Bytes<MemoryRegionAddress, E = Error>. The Bytes trait must be implemented.
  • GuestMemory has a generic implementation of Bytes<GuestAddress>.

Types:

  • GuestAddress: Address<u64>
  • MemoryRegionAddress: Address<u64>

Structs:

  • MmapRegion implements VolatileMemory
  • GuestRegionMmap implements Bytes<MemoryRegionAddress> + GuestMemoryRegion
  • GuestMemoryMmap implements GuestMemory
  • VolatileSlice implements Bytes<usize, E = volatile_memory::Error> + VolatileMemory