boot-qemu.py: Add support for passing through a folder to the guest

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 run virtiofsd, which is included with
most base QEMU packages. 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 ClangBuiltLinux#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

Link: ClangBuiltLinux#81
Signed-off-by: Nathan Chancellor <[email protected]>

.gitignore

@@ -1,2 +1,4 @@
 qemu-binaries/
 *.pyc
+shared/
+.vfsd.*

boot-qemu.py

@@ -2,6 +2,8 @@
 # pylint: disable=invalid-name
 
 import argparse
+import contextlib
+import grp
 import os
 from pathlib import Path
 import platform
@@ -13,6 +15,7 @@
 import utils
 
 base_folder = Path(__file__).resolve().parent
+shared_folder = base_folder.joinpath('shared')
 supported_architectures = [
     "arm", "arm32_v5", "arm32_v6", "arm32_v7", "arm64", "arm64be", "m68k",
     "mips", "mipsel", "ppc32", "ppc32_mac", "ppc64", "ppc64le", "riscv",
@@ -83,6 +86,12 @@ def parse_arguments():
         help=  # noqa: E251
         "Number of processors for virtual machine. By default, only machines spawned with KVM will use multiple vCPUS."
     )
+    parser.add_argument(
+        "--share-folder",
+        action='store_true',
+        help=  # noqa: E251
+        f"Share {shared_folder} with the guest using virtiofs (requires interactive, not supported with gdb)."
+    )
     parser.add_argument(
         "-t",
         "--timeout",
@@ -221,6 +230,7 @@ def setup_cfg(args):
         * interactive: Whether or not the user is going to be running the
                        machine interactively.
         * kernel_location: The full path to the kernel image or build folder.
+        * share_folder_with_guest: Share a folder on the host with a guest.
         * smp_requested: Whether or not the user specified a value with
                          '--smp'.
         * smp_value: The value to use with '-smp' (will be used when
@@ -246,6 +256,7 @@ def setup_cfg(args):
         "gdb": args.gdb,
         "gdb_bin": args.gdb_bin,
         "interactive": args.interactive or args.gdb,
+        "share_folder_with_guest": args.share_folder,
         "smp_requested": args.smp is not None,
         "smp_value": get_smp_value(args),
         "timeout": args.timeout,
@@ -733,8 +744,58 @@ def launch_qemu(cfg):
     gdb_bin = cfg["gdb_bin"]
     kernel_location = cfg["kernel_location"]
     qemu_cmd = cfg["qemu_cmd"]
+    share_folder_with_guest = cfg["share_folder_with_guest"]
     timeout = cfg["timeout"]
 
+    if share_folder_with_guest and not interactive:
+        utils.yellow(
+            'Shared folder requested without an interactive session, ignoring...'
+        )
+        share_folder_with_guest = False
+    if share_folder_with_guest and gdb:
+        utils.yellow(
+            'Shared folder requested during a debugging session, ignoring...')
+        share_folder_with_guest = False
+
+    if share_folder_with_guest:
+        shared_folder.mkdir(exist_ok=True, parents=True)
+
+        # If shared folder was requested, we need to search for virtiofsd in
+        # certain known locations.
+        qemu_prefix = Path(qemu_cmd[0]).resolve().parent.parent
+        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)
+
+        if not (sudo := shutil.which('sudo')):
+            raise Exception(
+                '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)
+
+        virtiofsd_log = base_folder.joinpath('.vfsd.log')
+        virtiofsd_mem = base_folder.joinpath('.vfsd.mem')
+        virtiofsd_socket = base_folder.joinpath('.vfsd.sock')
+        virtiofsd_cmd = [
+            sudo,
+            virtiofsd,
+            f"--socket-group={grp.getgrgid(os.getgid()).gr_name}",
+            f"--socket-path={virtiofsd_socket}",
+            '-o', f"source={shared_folder}",
+            '-o', 'cache=always',
+        ]  # yapf: disable
+
+        qemu_mem = qemu_cmd[qemu_cmd.index('-m') + 1]
+        qemu_cmd += [
+            '-chardev', f"socket,id=char0,path={virtiofsd_socket}",
+            '-device', 'vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=shared',
+            '-object', f"memory-backend-file,id=shm,mem-path={virtiofsd_mem},share=on,size={qemu_mem}",
+            '-numa', 'node,memdev=shm',
+        ]  # yapf: disable
+
     # Print information about the QEMU binary
     pretty_print_qemu_info(qemu_cmd[0])
 
@@ -772,14 +833,31 @@ def launch_qemu(cfg):
             qemu_cmd = timeout_cmd + stdbuf_cmd + qemu_cmd
 
         pretty_print_qemu_cmd(qemu_cmd)
-        try:
-            subprocess.run(qemu_cmd, check=True)
-        except subprocess.CalledProcessError as ex:
-            if ex.returncode == 124:
-                utils.red("ERROR: QEMU timed out!")
-            else:
-                utils.red("ERROR: QEMU did not exit cleanly!")
-            sys.exit(ex.returncode)
+        null_cm = contextlib.nullcontext()
+        with open(virtiofsd_log, 'w', encoding='utf-8') if share_folder_with_guest else null_cm as vfsd_log, \
+             subprocess.Popen(virtiofsd_cmd, stderr=vfsd_log, stdout=vfsd_log) if share_folder_with_guest else null_cm as vfsd_process:
+            try:
+                subprocess.run(qemu_cmd, check=True)
+            except subprocess.CalledProcessError as ex:
+                if ex.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_process and vfsd_process.poll():
+                        vfsd_log_txt = virtiofsd_log.read_text(
+                            encoding='utf-8')
+                        raise Exception(
+                            f"virtiofsd failed with: {vfsd_log_txt}") from ex
+                sys.exit(ex.returncode)
+            finally:
+                if vfsd_process:
+                    vfsd_process.kill()
+                    # Delete the memory to save space, it does not have to be
+                    # persistent
+                    virtiofsd_mem.unlink(missing_ok=True)
 
 
 if __name__ == '__main__':