virtfs root filesystem support

Qemu/KVM provides virtfs, a paravirtualised filesystem that is
implemented by running the Plan 9 folder sharing protocol over
virtio.

Make booting with root=virtfs:foobar use the virtfs filesystem
with mount tag 'foobar' as root filesystem, to allow booting
virtual machines off virtfs.

Note that this only handles 9p over virtio (i.e. virtfs), and
doesn't attempt to handle mounting 9p filesystems over TCP/IP,
for example.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>

dracut.spec

@@ -270,6 +270,7 @@ rm -rf $RPM_BUILD_ROOT
 %{dracutlibdir}/modules.d/95zfcp
 %{dracutlibdir}/modules.d/95terminfo
 %{dracutlibdir}/modules.d/95udev-rules
+%{dracutlibdir}/modules.d/95virtfs
 %{dracutlibdir}/modules.d/96securityfs
 %{dracutlibdir}/modules.d/97biosdevname
 %{dracutlibdir}/modules.d/97masterkey

modules.d/95virtfs/module-setup.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+# -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
+# ex: ts=8 sw=4 sts=4 et filetype=sh
+
+check() {
+    [[ $hostonly ]] || [[ $mount_needs ]] && {
+        for fs in ${host_fs_types[@]}; do
+            strstr "$fs" "\|9p" && return 0
+        done
+        return 1
+    }
+
+    return 0
+}
+
+depends() {
+    return 0
+}
+
+installkernel() {
+    instmods 9p 9pnet_virtio
+}
+
+install() {
+    inst_hook cmdline 95 "$moddir/parse-virtfs.sh"
+    inst_hook mount 99 "$moddir/mount-virtfs.sh"
+}

modules.d/95virtfs/mount-virtfs.sh

@@ -0,0 +1,75 @@
+#!/bin/sh
+# -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
+# ex: ts=8 sw=4 sts=4 et filetype=sh
+
+type getarg >/dev/null 2>&1 || . /lib/dracut-lib.sh
+
+filter_rootopts() {
+    rootopts=$1
+    # strip ro and rw options
+    local OLDIFS="$IFS"
+    IFS=,
+    set -- $rootopts
+    IFS="$OLDIFS"
+    local v
+    while [ $# -gt 0 ]; do
+        case $1 in
+            rw|ro);;
+            defaults);;
+            *)
+                v="$v,${1}";;
+        esac
+        shift
+    done
+    rootopts=${v#,}
+    echo $rootopts
+}
+
+mount_root() {
+    local _ret
+
+    rootfs="9p"
+    rflags="trans=virtio,version=9p2000.L"
+
+    modprobe 9pnet_virtio
+
+    mount -t ${rootfs} -o "$rflags",ro "${root#virtfs:}" "$NEWROOT"
+
+    rootopts=
+    if getargbool 1 rd.fstab -n rd_NO_FSTAB \
+        && ! getarg rootflags \
+        && [ -f "$NEWROOT/etc/fstab" ] \
+        && ! [ -L "$NEWROOT/etc/fstab" ]; then
+        # if $NEWROOT/etc/fstab contains special mount options for
+        # the root filesystem,
+        # remount it with the proper options
+        rootopts="defaults"
+        while read dev mp fs opts rest; do
+            # skip comments
+            [ "${dev%%#*}" != "$dev" ] && continue
+
+            if [ "$mp" = "/" ]; then
+                rootopts=$opts
+                break
+            fi
+        done < "$NEWROOT/etc/fstab"
+
+        rootopts=$(filter_rootopts $rootopts)
+    fi
+
+    # we want rootflags (rflags) to take precedence so prepend rootopts to
+    # them; rflags is guaranteed to not be empty
+    rflags="${rootopts:+"${rootopts},"}${rflags}"
+
+    umount "$NEWROOT"
+
+    info "Remounting ${root#virtfs:} with -o ${rflags}"
+    mount -t ${rootfs} -o "$rflags" "${root#virtfs:}" "$NEWROOT" 2>&1 | vinfo
+
+    [ -f "$NEWROOT"/forcefsck ] && rm -f "$NEWROOT"/forcefsck 2>/dev/null
+    [ -f "$NEWROOT"/.autofsck ] && rm -f "$NEWROOT"/.autofsck 2>/dev/null
+}
+
+if [ -n "$root" -a -z "${root%%virtfs:*}" ]; then
+    mount_root
+fi

modules.d/95virtfs/parse-virtfs.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+# -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
+# ex: ts=8 sw=4 sts=4 et filetype=sh
+
+if [ "${root%%:*}" = "virtfs" ] ; then
+    modprobe 9pnet_virtio
+
+    rootok=1
+fi