Multi-process QEMU

This document describes how to configure and use multi-process qemu. For the design document refer to docs/devel/qemu-multiprocess.

1) Configuration

multi-process is enabled by default for targets that enable KVM

2) Usage

Multi-process QEMU requires an orchestrator to launch.

Following is a description of command-line used to launch mpqemu.

  • Orchestrator:

    • The Orchestrator creates a unix socketpair
    • It launches the remote process and passes one of the sockets to it via command-line.
    • It then launches QEMU and specifies the other socket as an option to the Proxy device object
  • Remote Process:

    • QEMU can enter remote process mode by using the “remote” machine option.

    • The orchestrator creates a “remote-object” with details about the device and the file descriptor for the device

    • The remaining options are no different from how one launches QEMU with devices.

    • Example command-line for the remote process is as follows:

      /usr/bin/qemu-system-x86_64 -machine x-remote -device lsi53c895a,id=lsi0 -drive id=drive_image2,file=/build/ol7-nvme-test-1.qcow2 -device scsi-hd,id=drive2,drive=drive_image2,bus=lsi0.0,scsi-id=0 -object x-remote-object,id=robj1,devid=lsi1,fd=4,

  • QEMU:

    • Since parts of the RAM are shared between QEMU & remote process, a memory-backend-memfd is required to facilitate this, as follows:

      -object memory-backend-memfd,id=mem,size=2G

    • A “x-pci-proxy-dev” device is created for each of the PCI devices emulated in the remote process. A “socket” sub-option specifies the other end of unix channel created by orchestrator. The “id” sub-option must be specified and should be the same as the “id” specified for the remote PCI device

    • Example commandline for QEMU is as follows:

      -device x-pci-proxy-dev,id=lsi0,socket=3