feat: add virtiofs support for Linux guests

Automatically use virtiofsd (vhost-user-fs-pci) instead of 9p when
virtiofsd is present on the host and the guest OS is Linux. Falls back
silently to 9p when virtiofsd is unavailable. Updates README to document
the new capability.

Author: codemedic

README.md

@@ -47,6 +47,7 @@ required to run the virtual machines.
 - **Nearly 1000 operating system editions are supported!**
 - Full SPICE support including host/guest clipboard sharing
 - VirtIO-webdavd file sharing for Linux and Windows guests
+- VirtIO-fs file sharing for Linux guests (*automatically preferred over 9p when `virtiofsd` is installed on the host*)
 - VirtIO-9p file sharing for Linux and macOS guests
 - [QEMU Guest Agent
   support](https://wiki.qemu.org/Features/GuestAgent); provides access

quickemu

@@ -1544,15 +1544,16 @@ function configure_file_sharing() {
             *) echo " - WebDAV:   On guest: dav://localhost:9843/";;
         esac
 
-        # 9P
+        # virtiofs or 9p depending on host capability
         if [ "${guest_os}" != "windows" ] || [ "${guest_os}" == "windows-server" ]; then
-            echo -n " - 9P:       On guest: "
-            if [ "${guest_os}" == "linux" ]; then
-                echo "sudo mount -t 9p -o trans=virtio,version=9p2000.L,msize=104857600 ${PUBLIC_TAG} ~/$(basename "${PUBLIC}")"
+            if [ "${guest_os}" == "linux" ] && [ -n "${VIRTIOFSD}" ]; then
+                echo " - virtiofs: On guest: sudo mount -t virtiofs ${PUBLIC_TAG} ~/$(basename "${PUBLIC}")"
+            elif [ "${guest_os}" == "linux" ]; then
+                echo " - 9P:       On guest: sudo mount -t 9p -o trans=virtio,version=9p2000.L,msize=104857600 ${PUBLIC_TAG} ~/$(basename "${PUBLIC}")"
             elif [ "${guest_os}" == "macos" ]; then
                 # PUBLICSHARE needs to be world writeable for seamless integration with
                 # macOS. Test if it is world writeable, and prompt what to do if not.
-                echo "sudo mount_9p ${PUBLIC_TAG}"
+                echo " - 9P:       On guest: sudo mount_9p ${PUBLIC_TAG}"
                 if [ "${PUBLIC_PERMS}" != "drwxrwxrwx" ]; then
                     echo " - 9P:       On host:  chmod 777 ${PUBLIC}"
                     echo "             Required for macOS integration 👆"
@@ -1616,6 +1617,37 @@ function configure_cpu_pinning() {
     echo " - CPU Pinning: Bind guest cores to host cores (${GUEST_CPUS} -> ${CPU_PINNING})"
 }
 
+function start_virtiofsd() {
+    # Start virtiofsd as a background daemon and record its PID so it can be
+    # cleaned up when the VM exits. The socket path is placed alongside other
+    # VM runtime files in VMDIR.
+    if [ -z "${VIRTIOFSD}" ]; then
+        return
+    fi
+
+    VIRTIOFSD_SOCKET="${VMDIR}/${VMNAME}.virtiofsd-sock"
+    local virtiofsd_args=(
+        --socket-path="${VIRTIOFSD_SOCKET}"
+        --shared-dir="${PUBLIC}"
+        --announce-submounts
+    )
+
+    echo "${VIRTIOFSD} ${virtiofsd_args[*]} &" >> "${VMDIR}/${VMNAME}.sh"
+    ${VIRTIOFSD} "${virtiofsd_args[@]}" >> "${VMDIR}/${VMNAME}.log" 2>&1 &
+    VIRTIOFSD_PID=$!
+    sleep 0.25
+
+    if ! kill -0 "${VIRTIOFSD_PID}" 2>/dev/null; then
+        echo " - WARNING! virtiofsd failed to start; falling back to 9p."
+        VIRTIOFSD=""
+        VIRTIOFSD_SOCKET=""
+        VIRTIOFSD_PID=""
+        return
+    fi
+
+    echo " - virtiofsd: ${VIRTIOFSD_SOCKET} (${VIRTIOFSD_PID})"
+}
+
 function vm_boot() {
     AUDIO_DEV=""
     BALLOON="-device virtio-balloon"
@@ -2053,12 +2085,23 @@ function vm_boot() {
         fi
     fi
 
+    # File sharing: prefer virtiofs (shared memory, lower latency) over 9p when
+    # virtiofsd is available; virtiofsd must already be running at this point.
     # https://wiki.qemu.org/Documentation/9psetup
     # https://askubuntu.com/questions/772784/9p-libvirt-qemu-share-modes
     if [ "${guest_os}" != "windows" ] || [ "${guest_os}" == "windows-server" ] && [ -n "${PUBLIC}" ]; then
-        # shellcheck disable=SC2054
-        args+=(-fsdev local,id=fsdev0,path="${PUBLIC}",security_model=mapped-xattr
-            -device virtio-9p-pci,fsdev=fsdev0,mount_tag="${PUBLIC_TAG}")
+        if [ -n "${VIRTIOFSD_SOCKET}" ]; then
+            # virtiofs requires a shared-memory backend; the size mirrors the VM RAM.
+            # shellcheck disable=SC2054
+            args+=(-object "memory-backend-file,id=mem,size=${RAM_VM},mem-path=/dev/shm,share=on"
+                -numa node,memdev=mem
+                -chardev "socket,id=char0,path=${VIRTIOFSD_SOCKET}"
+                -device "vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=${PUBLIC_TAG}")
+        else
+            # shellcheck disable=SC2054
+            args+=(-fsdev local,id=fsdev0,path="${PUBLIC}",security_model=mapped-xattr
+                -device virtio-9p-pci,fsdev=fsdev0,mount_tag="${PUBLIC_TAG}")
+        fi
     fi
 
     if [ -n "${USB_PASSTHROUGH}" ]; then
@@ -2469,6 +2512,16 @@ function fileshare_param_check() {
             PUBLIC_PERMS=$(${STAT}  -c "%A" "${PUBLIC}")
         fi
     fi
+
+    # Prefer virtiofs over 9p when virtiofsd is available and the guest is Linux.
+    # virtiofs uses shared memory rather than a transport protocol, giving much
+    # lower latency and higher throughput than 9p.
+    if [ -n "${PUBLIC}" ] && [ "${guest_os}" == "linux" ]; then
+        VIRTIOFSD=$(command -v virtiofsd 2>/dev/null || echo "/usr/lib/qemu/virtiofsd")
+        if [ ! -x "${VIRTIOFSD}" ]; then
+            VIRTIOFSD=""
+        fi
+    fi
 }
 
 function parse_ports_from_file {
@@ -2579,6 +2632,9 @@ MONITOR_CMD=""
 PUBLIC=""
 PUBLIC_PERMS=""
 PUBLIC_TAG=""
+VIRTIOFSD=""
+VIRTIOFSD_PID=""
+VIRTIOFSD_SOCKET=""
 SHORTCUT_OPTIONS=""
 SNAPSHOT_ACTION=""
 SNAPSHOT_TAG=""
@@ -2903,6 +2959,7 @@ viewer_param_check
 fileshare_param_check
 
 if [ -z "${VM_PID}" ]; then
+    start_virtiofsd
     vm_boot
     start_viewer
     # If the VM being started is an uninstalled Windows VM then auto-skip the press-any key prompt.