Lecture: I/O virtualization

• review physical I/O from 451
  • examples: abstract, photo
  • control
    • port-mapped I/O (PIO)
    • memroy-mapped I/O (MMIO)
  • data transfer: direct memory access (DMA)
    • example: NVMe and NIC in JOS
    • ring buffers
  • interrupts
    • see Figure 10-2 of Intel SDM 3 about local APICs and I/O APIC
    • PIC, local APIC, I/O APIC, message signaled interrupts (MSI)
    • IDT
  • example: how an app communicates to the network
    • send & recv
    • consider two processes
      • how does the OS decide which one to deliver packets to
      • what’s the performance overhead
      • what about two virtual machines
      • how would you optimize this
• virtualization goals
  • multiplexing: mutiple virtual devices share the same physical device
  • performance: see the Linux I/O path
• virtualization approaches
  • emulation
  • paravirtualization
  • hardware virtualization
• emulation
  • example: emulate a disk on top of a Linux host/file system
  • example: emulate an e1000 NIC
  • how to intercept PIO, MMIO by guest?
  • what’s the overhead of every PIO/MMIO access
    • guest -> host kernel (KVM) -> host user (QEMU) -> …
    • also see Figure 6.7 of the textbook
• I/O paravirtualization
  • problem: the interface of physical devices was not designed for virtualization
  • goal: interface redesign for performance
    • reduce VM exits
    • allow backend drivers to reduce switches to QEMU
  • trade-off: need drivers in guest (and possibly host)
  • VirtIO for KVM
  • example from Table 6.2 of the textbook: 22x throughput (virtio-net vs e1000)
• hardware support
  • reduce VM exits
  • examples
    • IOMMU
      • assign devices to guests
      • Figure 2-3, Example Virtualization Usage of DMA Remapping, VT-d spec
      • Figure 6.15: IOVA translation with the Intel IOMMU, textbook
    • interrupt posting
    • single root I/O virtualization (SR-IOV)