The Problem #

I'm trying to launch a Debian-based VM using a custom-compiled QEMU from the harddoom branch with this command:

qemu-system-x86_64 \
 -drive file=zso_cow.img,if=virtio \
 -enable-kvm \
 -smp 2 \
 -net nic,model=virtio -net user \
 -m 1G -balloon virtio \
 -fsdev local,id=hshare,path=hshare/,security_model=none -device virtio-9p-pci,fsdev=hshare,mount_tag=hshare \
 -chardev stdio,id=cons,signal=off -device virtio-serial-pci -device virtconsole,chardev=cons \
 -device harddoom

The VM image zso_cow.img was created with:

qemu-img create -f qcow2 -o backing_file=zso.img zso_cow.img

But I hit this error:

qemu-system-x86_64: -device virtio-9p-pci,fsdev=hshare,mount_tag=hshare: 'virtio-9p-pci' is not a valid device model name

Ubuntu's repo-installed QEMU runs this command fine, but I need to use the harddoom-compiled version. Removing the virtio-9p-pci device breaks my shared folder setup.

My configure flags for compiling QEMU were:

--target-list=i386-softmmu,x86_64-softmmu --python=$(which python2) --audio-drv-list=alsa,pa

The Solution #

1. Recompile QEMU with VirtFS Enabled #

The root issue is that your custom build didn't include support for the 9p virtio filesystem device. You need to explicitly enable this feature during configuration:

1. Clean your existing build (optional but recommended):

   make clean
   

2. Re-run the configure script with the --enable-virtfs flag:

   ./configure --target-list=i386-softmmu,x86_64-softmmu --python=$(which python2) --audio-drv-list=alsa,pa --enable-virtfs
   

3. Recompile and install:

   make -j$(nproc)  # Use all CPU cores for faster build
   sudo make install
   

2. Verify the Device is Now Available #

After recompiling, check if virtio-9p-pci is listed in QEMU's supported devices:

qemu-system-x86_64 -device help | grep virtio-9p

If you see virtio-9p-pci in the output, your build now includes the necessary support.

3. Fallback: Try the Legacy Device Name #

If for some reason --enable-virtfs doesn't add virtio-9p-pci, you can try the older device name virtio-9p-device as a replacement in your launch command:

-fsdev local,id=hshare,path=hshare/,security_model=none -device virtio-9p-device,fsdev=hshare,mount_tag=hshare

This is less ideal, but works with some older QEMU configurations.

Why This Works #

Ubuntu's pre-built QEMU packages include VirtFS support by default, but when compiling from source, you have to enable it explicitly. The --enable-virtfs flag tells the build system to include the 9p virtio device drivers, which adds the virtio-9p-pci device you need for shared folders.

内容的提问来源于stack exchange，提问作者Michał Piotr Stankiewicz