pkgs.vmTools.vmRunCommand

The following is the current implementation of this function.

qemuCommand:
    writeText "vm-run" ''
      ${coreutils}/bin/mkdir xchg
      export > xchg/saved-env

      if [ -f "''${NIX_ATTRS_SH_FILE-}" ]; then
        ${coreutils}/bin/cp $NIX_ATTRS_JSON_FILE $NIX_ATTRS_SH_FILE xchg
        source "$NIX_ATTRS_SH_FILE"
      fi
      source $stdenv/setup

      eval "$preVM"

      if [ "$enableParallelBuilding" = 1 ]; then
        QEMU_NR_VCPUS=0
        if [ ''${NIX_BUILD_CORES:-0} = 0 ]; then
          QEMU_NR_VCPUS="$(nproc)"
        else
          QEMU_NR_VCPUS="$NIX_BUILD_CORES"
        fi
        # qemu only supports 255 vCPUs (see error from `qemu-system-x86_64 -smp 256`)
        if [ "$QEMU_NR_VCPUS" -gt 255 ]; then
          QEMU_NR_VCPUS=255
        fi
        QEMU_OPTS+=" -smp cpus=$QEMU_NR_VCPUS"
      fi

      # Write the command to start the VM to a file so that the user can
      # debug inside the VM if the build fails (when Nix is called with
      # the -K option to preserve the temporary build directory).
      ${coreutils}/bin/cat > ./run-vm <<EOF
      #! ${bash}/bin/sh
      ''${diskImage:+diskImage=$diskImage}
      # GitHub Actions runners seems to not allow installing seccomp filter: https://github.com/rcambrj/nix-pi-loader/issues/1#issuecomment-2605497516
      # Since we are running in a sandbox already, the difference between seccomp and none is minimal
      ${virtiofsd}/bin/virtiofsd --xattr --socket-path virtio-store.sock --sandbox none --seccomp none --shared-dir "${storeDir}" &
      ${virtiofsd}/bin/virtiofsd --xattr --socket-path virtio-xchg.sock --sandbox none --seccomp none --shared-dir xchg &

      # Wait until virtiofsd has created these sockets to avoid race condition.
      until [[ -e virtio-store.sock ]]; do ${coreutils}/bin/sleep 0.1; done
      until [[ -e virtio-xchg.sock ]]; do ${coreutils}/bin/sleep 0.1; done

      ${qemuCommand}
      EOF

      ${coreutils}/bin/chmod +x ./run-vm
      source ./run-vm

      if ! test -e xchg/in-vm-exit; then
        echo "Virtual machine didn't produce an exit code."
        exit 1
      fi

      exitCode="$(${coreutils}/bin/cat xchg/in-vm-exit)"
      if [ "$exitCode" != "0" ]; then
        exit "$exitCode"
      fi

      eval "$postVM"
    ''