boot-qemu.py: Add support for mounting a folder into the guest via virtiofs

virtiofs, available in QEMU 5.2 or newer and Linux guests 5.4 or newer,
is a more modern way to pass local folders along to QEMU, as it takes
advantage of the fact that the folders are on the same machine as the
hypervisor.

To use virtiofs, we first need to find and run virtiofsd, which has two
different implementations: a C implementation included with QEMU up until
8.0 (available on most distros) and a standalone Rust implementation
available on GitLab (not packaged on many distros but easy to build and
install). Once we find it, we run it in the background and connect to it
using some QEMU parameters, which were shamelessly taken from the
official virtiofs website:

https://virtio-fs.gitlab.io/howto-qemu.html

To use it within the guest (you can use a different path than
/mnt/shared but 'mount -t virtio shared' must be used):

  # mkdir /mnt/shared
  # mount -t virtiofs shared /mnt/shared
  # echo "$(uname -a)" >/mnt/shared/foo

On the host:

  $ cat shared/foo
  Linux (none) 6.1.0-rc8-next-20221207 #2 SMP PREEMPT Wed Dec  7 14:56:03 MST 2022 aarch64 GNU/Linux

This does require guest kernel support (CONFIG_VIRTIO_FS=y), otherwise
it will not work inside the guest:

  / # mount -t virtiofs shared /mnt/shared
  mount: mounting shared on /mnt/shared failed: No such device

Closes: #81
Link: https://gitlab.com/virtio-fs/virtiofsd
Signed-off-by: Nathan Chancellor <[email protected]>

Author: nathanchance

.gitignore

@@ -1,3 +1,5 @@
 images/
 qemu-binaries/
 *.pyc
+shared/
+.vfsd.*

boot-qemu.py

@@ -3,6 +3,7 @@
 
 from argparse import ArgumentParser
 import contextlib
+import grp
 import os
 from pathlib import Path
 import platform
@@ -14,6 +15,7 @@
 
 import utils
 
+SHARED_FOLDER = Path(utils.BOOT_UTILS, 'shared')
 SUPPORTED_ARCHES = [
     'arm',
     'arm32_v5',
@@ -40,7 +42,7 @@ class QEMURunner:
 
     def __init__(self):
 
-        # Properties that can be adjusted by the user or class
+        # Properties that can be adjusted by the user or class (keep alphabetized if possible)
         self.cmdline = []
         self.efi = False
         self.gdb = False
@@ -51,14 +53,15 @@ def __init__(self):
         self.kernel_config = None
         self.kernel_dir = None
         self.memory = '512m'
+        self.share_folder_with_guest = False
+        self.smp = 0
         self.supports_efi = False
+        self.timeout = ''
         # It may be tempting to use self.use_kvm during initialization of
         # subclasses to set certain properties but the user can explicitly opt
         # out of KVM after instantiation, so any decisions based on it should
         # be confined to run().
         self.use_kvm = False
-        self.smp = 0
-        self.timeout = ''
 
         self._default_kernel_path = None
         self._dtbs = []
@@ -72,6 +75,14 @@ def __init__(self):
             '-nodefaults',
         ]  # yapf: disable
         self._qemu_path = None
+        self._vfsd_conf = {
+            'cmd': [],
+            'files': {
+                'log': Path(utils.BOOT_UTILS, '.vfsd.log'),
+                'mem': Path(utils.BOOT_UTILS, '.vfsd.mem'),
+                'sock': Path(utils.BOOT_UTILS, '.vfsd.sock'),
+            },
+        }
 
     def _find_dtb(self):
         if not self._dtbs:
@@ -173,13 +184,75 @@ def _get_qemu_ver_tuple(self):
     def _have_dev_kvm_access(self):
         return os.access('/dev/kvm', os.R_OK | os.W_OK)
 
+    def _prepare_for_shared_folder(self):
+        if self._get_kernel_config_val('CONFIG_VIRTIO_FS') != 'y':
+            utils.yellow(
+                'CONFIG_VIRTIO_FS may not be enabled in your configuration, shared folder may not work...'
+            )
+
+        # Print information about using shared folder
+        utils.green('To mount shared folder in guest (e.g. to /mnt/shared):')
+        utils.green('\t/ # mkdir /mnt/shared')
+        utils.green('\t/ # mount -t virtiofs shared /mnt/shared')
+
+        SHARED_FOLDER.mkdir(exist_ok=True, parents=True)
+
+        # Make sure sudo is available and we have permission to use it
+        if not (sudo := shutil.which('sudo')):
+            raise FileNotFoundError(
+                'sudo is required to use virtiofsd but it could not be found!')
+        utils.green(
+            'Requesting sudo permission to run virtiofsd in the background...')
+        subprocess.run([sudo, 'true'], check=True)
+
+        # There are two implementations of virtiofsd. The original C
+        # implementation was bundled and built with QEMU up until 8.0, where it
+        # was removed after being deprecated in 7.0:
+        #
+        # https://lore.kernel.org/[email protected]/
+        #
+        # The standalone Rust implementation is preferred now, which should be
+        # available in PATH. If it is not available, see if there is a C
+        # implementation available in QEMU's prefix.
+        if not (virtiofsd := shutil.which('virtiofsd')):
+            utils.yellow(
+                'Could not find Rust implementation of virtiofsd (https://gitlab.com/virtio-fs/virtiofsd), searching for old C implementation...'
+            )
+
+            qemu_prefix = self._qemu_path.resolve().parents[1]
+            virtiofsd_locations = [
+                Path('libexec/virtiofsd'),  # Default QEMU installation, Fedora
+                Path('lib/qemu/virtiofsd'),  # Arch Linux, Debian, Ubuntu
+            ]
+            virtiofsd = utils.find_first_file(qemu_prefix, virtiofsd_locations)
+
+        # Prepare QEMU arguments
+        self._qemu_args += [
+            '-chardev', f"socket,id=char0,path={self._vfsd_conf['files']['sock']}",
+            '-device', 'vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=shared',
+            '-object', f"memory-backend-file,id=shm,mem-path={self._vfsd_conf['files']['mem']},share=on,size={self._ram}",
+            '-numa', 'node,memdev=shm',
+        ]  # yapf: disable
+
+        self._vfsd_conf['cmd'] = [
+            sudo,
+            virtiofsd,
+            f"--socket-group={grp.getgrgid(os.getgid()).gr_name}",
+            f"--socket-path={self._vfsd_conf['files']['sock']}",
+            '-o', f"source={SHARED_FOLDER}",
+            '-o', 'cache=always',
+        ]  # yapf: disable
+
     def _prepare_initrd(self):
         if not self._initrd_arch:
             raise RuntimeError('No initrd architecture specified?')
         return utils.prepare_initrd(self._initrd_arch,
                                     gh_json_file=self.gh_json_file)
 
     def _run_fg(self):
+        if self.share_folder_with_guest:
+            self._prepare_for_shared_folder()
+
         # Pretty print and run QEMU command
         qemu_cmd = []
 
@@ -192,15 +265,32 @@ def _run_fg(self):
 
         qemu_cmd += [self._qemu_path, *self._qemu_args]
 
-        print(f"$ {' '.join(shlex.quote(str(elem)) for elem in qemu_cmd)}")
-        try:
-            subprocess.run(qemu_cmd, check=True)
-        except subprocess.CalledProcessError as err:
-            if err.returncode == 124:
-                utils.red("ERROR: QEMU timed out!")
-            else:
-                utils.red("ERROR: QEMU did not exit cleanly!")
-            sys.exit(err.returncode)
+        print(f"\n$ {' '.join(shlex.quote(str(elem)) for elem in qemu_cmd)}")
+        null_cm = contextlib.nullcontext()
+        with self._vfsd_conf['files']['log'].open('w', encoding='utf-8') if self.share_folder_with_guest else null_cm as vfsd_log, \
+            subprocess.Popen(self._vfsd_conf['cmd'], stderr=vfsd_log, stdout=vfsd_log) if self.share_folder_with_guest else null_cm as vfsd_proc:
+            try:
+                subprocess.run(qemu_cmd, check=True)
+            except subprocess.CalledProcessError as err:
+                if err.returncode == 124:
+                    utils.red("ERROR: QEMU timed out!")
+                else:
+                    utils.red("ERROR: QEMU did not exit cleanly!")
+                    # If virtiofsd is dead, it is pretty likely that it was the
+                    # cause of QEMU failing so add to the existing exception using
+                    # 'from'.
+                    if vfsd_proc and vfsd_proc.poll():
+                        # yapf: disable
+                        vfsd_log_txt = self._vfsd_conf['files']['log'].read_text(encoding='utf-8')
+                        raise RuntimeError(f"virtiofsd failed with: {vfsd_log_txt}") from err
+                        # yapf: enable
+                sys.exit(err.returncode)
+            finally:
+                if vfsd_proc:
+                    vfsd_proc.kill()
+                    # Delete the memory to save space, it does not have to be
+                    # persistent
+                    self._vfsd_conf['files']['mem'].unlink(missing_ok=True)
 
     def _run_gdb(self):
         qemu_cmd = [self._qemu_path, *self._qemu_args]
@@ -846,6 +936,12 @@ def parse_arguments():
         help=
         'Number of processors for virtual machine (default: only KVM machines will use multiple vCPUs.)',
     )
+    parser.add_argument(
+        '--share-folder-with-guest',
+        action='store_true',
+        help=
+        f"Share {SHARED_FOLDER} with the guest using virtiofs (requires interactive, not supported with gdb).",
+    )
     parser.add_argument('-t',
                         '--timeout',
                         default='3m',
@@ -918,6 +1014,18 @@ def parse_arguments():
     if args.no_kvm:
         runner.use_kvm = False
 
+    if args.share_folder_with_guest:
+        if args.gdb:
+            utils.yellow(
+                'Shared folder requested during a debugging session, ignoring...'
+            )
+        elif not args.interactive:
+            utils.yellow(
+                'Shared folder requested without an interactive session, ignoring...'
+            )
+        else:
+            runner.share_folder_with_guest = True
+
     if args.smp:
         runner.smp = args.smp