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I'd like to do what is mentioned here: https://wiki.archlinux.org/index.php/QEMU#Mounting_a_partition_of_the_guest_on_the_host - essentially mount a partition directly on the VM - this is so I can use mmap on the mount - (which 9p does not seem to support).

However I'm not sure of the relevant config for the nix module(?) here https://github.com/NixOS/nixpkgs/blob/b0df5a6816b644c2e0b6ebcde6ad0784b07205e0/nixos/modules/virtualisation/qemu-vm.nix to configure this. I'm actually using https://github.com/Mic92/nixos-shell though which in turn seems to use the referenced module/file.

Trying something like:

    extraMounts = {
      "/test" = {
        target = /dev/sdc;
        cache = "none";
      };
    };

results in an error of:

qemu-system-x86_64: -virtfs local,path=/dev/sdc,security_model=none,mount_tag=a457b9c00b7152b02ceea27e7fe1a07: cannot initialize fsdev 'a457b9c00b7152b02ceea27e7fe1a07': failed to open '/dev/sdc': Not a directory

It seems I need to set a -hda parameter, but I don't see any option in the nix module for this.

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Solution found here: https://github.com/Mic92/nixos-shell/issues/14

I had to set an environment variable: QEMU_OPTS="-hdc /dev/disk/by-uuid/836d83ab-cba5-4eb4-a7c8-09911cd67403"

Which gets referenced here:

      # Start QEMU.
      exec ${qemuBinary qemu} \
          -name ${vmName} \
          -m ${toString config.virtualisation.memorySize} \
          -smp ${toString config.virtualisation.cores} \
          -device virtio-rng-pci \
          ${concatStringsSep " " config.virtualisation.qemu.networkingOptions} \
          -virtfs local,path=/nix/store,security_model=none,mount_tag=store \
          -virtfs local,path=$TMPDIR/xchg,security_model=none,mount_tag=xchg \
          -virtfs local,path=''${SHARED_DIR:-$TMPDIR/xchg},security_model=none,mount_tag=shared \
          ${drivesCmdLine config.virtualisation.qemu.drives} \
          ${toString config.virtualisation.qemu.options} \
          $QEMU_OPTS \
          "$@"

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