guestfs-internals - architecture and internals of libguestfs
This manual page is for hackers who want to understand how libguestfs works
internally. This is just a description of how libguestfs works now, and it may
change at any time in the future.
内部的に、libguestfs は
qemu(1)
を使用してアプライアンス（特別な形式の小さな仮想マシン）を実行することにより実装されます。QEMU
はメインプログラムの子プロセスとして実行します。
┌───────────────────┐
│ main program │
│ │
│ │ child process / appliance
│ │ ┌──────────────────────────┐
│ │ │ qemu │
├───────────────────┤ RPC │ ┌─────────────────┐ │
│ libguestfs ◀╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍▶ guestfsd │ │
│ │ │ ├─────────────────┤ │
└───────────────────┘ │ │ Linux kernel │ │
│ └────────┬────────┘ │
└───────────────│──────────┘
│
│ virtio-scsi
┌──────┴──────┐
│ Device or │
│ disk image │
└─────────────┘
The library, linked to the main program, creates the child process and hence the
appliance in the "guestfs_launch" in
guestfs(3) function.
Inside the appliance is a Linux kernel and a complete stack of userspace tools
(such as LVM and ext2 programs) and a small controlling daemon called
"guestfsd". The library talks to "guestfsd" using remote
procedure calls (RPC). There is a mostly one-to-one correspondence between
libguestfs API calls and RPC calls to the daemon. Lastly the disk image(s) are
attached to the qemu process which translates device access by the
appliance’s Linux kernel into accesses to the image.
A common misunderstanding is that the appliance "is" the virtual
machine. Although the disk image you are attached to might also be used by
some virtual machine, libguestfs doesn't know or care about this. (But you
will care if both libguestfs’s qemu process and your virtual machine
are trying to update the disk image at the same time, since these usually
results in massive disk corruption).
libguestfs
は子プロセスをモデル化するために状態マシンを使用します:
|
guestfs_create / guestfs_create_flags
|
|
____V_____
/ \
| 設定 |
\__________/
^ ^ \
| \ \ guestfs_launch
| _\__V______
| / \
| | 起動中 |
| \___________/
| /
| guestfs_launch
| /
_____|____V
/ \
| 準備完了 |
\___________/
The normal transitions are (1) CONFIG (when the handle is created, but there is no child process), (2) LAUNCHING (when the child process is booting up), (3) READY meaning the appliance is up, actions can be issued to, and carried out by, the child process.
The guest may be killed by "guestfs_kill_subprocess" in
guestfs(3), or may die asynchronously at any time (eg. due to some
internal error), and that causes the state to transition back to CONFIG.
Configuration commands for qemu such as "guestfs_set_path" in
guestfs(3) can only be issued when in the CONFIG state.
The API offers one call that goes from CONFIG through LAUNCHING to READY.
"guestfs_launch" in
guestfs(3) blocks until the child process
is READY to accept commands (or until some failure or timeout).
"guestfs_launch" in
guestfs(3) internally moves the state
from CONFIG to LAUNCHING while it is running.
API actions such as "guestfs_mount" in
guestfs(3) can only be
issued when in the READY state. These API calls block waiting for the command
to be carried out. There are no non-blocking versions, and no way to issue
more than one command per handle at the same time.
Finally, the child process sends asynchronous messages back to the main program,
such as kernel log messages. You can register a callback to receive these
messages.
このプロセスは進化してきました。そして、進化し続けます。ここの記述は現在のバージョンの
libguestfs
にのみ対応していて、参考情報としてのみ提供されます。
以下に関係する段階に従うには
libguestfs
デバッグを有効にします（環境変数
"LIBGUESTFS_DEBUG=1"
を設定します）。
- アプライアンスを作成します
- "supermin --build" is invoked to create the
kernel, a small initrd and the appliance.
The appliance is cached in /var/tmp/.guestfs-<UID> (or in
another directory if "LIBGUESTFS_CACHEDIR" or "TMPDIR"
are set).
For a complete description of how the appliance is created and cached, read
the supermin(1) man page.
- QEMU
を開始してカーネルを起動します
- カーネルを起動するために
QEMU
が呼び出されます。
- initrd
を実行します
- "supermin --build" builds a small initrd. The
initrd is not the appliance. The purpose of the initrd is to load enough
kernel modules in order that the appliance itself can be mounted and
started.
The initrd is a cpio archive called
/var/tmp/.guestfs-<UID>/appliance.d/initrd.
initrd
が起動したとき、カーネルモジュールが読み込まれたことを示すこのようなメッセージが表示されます:
supermin: ext2 mini initrd starting up
supermin: mounting /sys
supermin: internal insmod libcrc32c.ko
supermin: internal insmod crc32c-intel.ko
- アプライアンスデバイスを検索およびマウントします
- The appliance is a sparse file containing an ext2
filesystem which contains a familiar (although reduced in size) Linux
operating system. It would normally be called
/var/tmp/.guestfs-<UID>/appliance.d/root.
The regular disks being inspected by libguestfs are the first devices
exposed by qemu (eg. as /dev/vda).
The last disk added to qemu is the appliance itself (eg. /dev/vdb if
there was only one regular disk).
Thus the final job of the initrd is to locate the appliance disk, mount it,
and switch root into the appliance, and run /init from the
appliance.
If this works successfully you will see messages such as:
supermin: picked /sys/block/vdb/dev as root device
supermin: creating /dev/root as block special 252:16
supermin: mounting new root on /root
supermin: chroot
Starting /init script ...
Note that "Starting /init script ..." indicates that the
appliance's init script is now running.
- アプライアンスを初期化します
- The appliance itself now initializes itself. This involves
starting certain processes like "udev", possibly printing some
debug information, and finally running the daemon
("guestfsd").
- デーモン
- Finally the daemon ("guestfsd") runs inside the
appliance. If it runs you should see:
verbose daemon enabled
The daemon expects to see a named virtio-serial port exposed by qemu and
connected on the other end to the library.
The daemon connects to this port (and hence to the library) and sends a four
byte message "GUESTFS_LAUNCH_FLAG", which initiates the
communication protocol (see below).
Don’t rely on using this protocol directly. This section documents how it
currently works, but it may change at any time.
The protocol used to talk between the library and the daemon running inside the
qemu virtual machine is a simple RPC mechanism built on top of XDR (RFC 1014,
RFC 1832, RFC 4506).
The detailed format of structures is in
common/protocol/guestfs_protocol.x (note: this file is automatically
generated).
There are two broad cases, ordinary functions that don’t have any
"FileIn" and "FileOut" parameters, which are handled with
very simple request/reply messages. Then there are functions that have any
"FileIn" or "FileOut" parameters, which use the same
request and reply messages, but they may also be followed by files sent using
a chunked encoding.
ORDINARY FUNCTIONS (NO FILEIN/FILEOUT PARAMS)
For ordinary functions, the request message is:
total length (header + arguments,
but not including the length word itself)
struct guestfs_message_header (encoded as XDR)
struct guestfs_<foo>_args (encoded as XDR)
The total length field allows the daemon to allocate a fixed size buffer into
which it slurps the rest of the message. As a result, the total length is
limited to "GUESTFS_MESSAGE_MAX" bytes (currently 4MB), which means
the effective size of any request is limited to somewhere under this size.
Note also that many functions don’t take any arguments, in which case the
"guestfs_
foo_args" is completely omitted.
The header contains the procedure number ("guestfs_proc") which is how
the receiver knows what type of args structure to expect, or none at all.
For functions that take optional arguments, the optional arguments are encoded
in the "guestfs_
foo_args" structure in the same way as
ordinary arguments. A bitmask in the header indicates which optional arguments
are meaningful. The bitmask is also checked to see if it contains bits set
which the daemon does not know about (eg. if more optional arguments were
added in a later version of the library), and this causes the call to be
rejected.
The reply message for ordinary functions is:
total length (header + ret,
but not including the length word itself)
struct guestfs_message_header (encoded as XDR)
struct guestfs_<foo>_ret (encoded as XDR)
As above the "guestfs_
foo_ret" structure may be completely
omitted for functions that return no formal return values.
As above the total length of the reply is limited to
"GUESTFS_MESSAGE_MAX".
In the case of an error, a flag is set in the header, and the reply message is
slightly changed:
total length (header + error,
but not including the length word itself)
struct guestfs_message_header (encoded as XDR)
struct guestfs_message_error (encoded as XDR)
"guestfs_message_error"
の構造は、文字列としてエラーメッセージを含みます。
FUNCTIONS THAT HAVE FILEIN PARAMETERS
A "FileIn" parameter indicates that we transfer a file
into the
guest. The normal request message is sent (see above). However this is
followed by a sequence of file chunks.
total length (header + arguments,
but not including the length word itself,
and not including the chunks)
struct guestfs_message_header (encoded as XDR)
struct guestfs_<foo>_args (encoded as XDR)
sequence of chunks for FileIn param #0
sequence of chunks for FileIn param #1 etc.
The "sequence of chunks" is:
length of chunk (not including length word itself)
struct guestfs_chunk (encoded as XDR)
length of chunk
struct guestfs_chunk (encoded as XDR)
...
length of chunk
struct guestfs_chunk (with data.data_len == 0)
The final chunk has the "data_len" field set to zero. Additionally a
flag is set in the final chunk to indicate either successful completion or
early cancellation.
At time of writing there are no functions that have more than one FileIn
parameter. However this is (theoretically) supported, by sending the sequence
of chunks for each FileIn parameter one after another (from left to right).
Both the library (sender)
and the daemon (receiver) may cancel the
transfer. The library does this by sending a chunk with a special flag set to
indicate cancellation. When the daemon sees this, it cancels the whole RPC,
does
not send any reply, and goes back to reading the next request.
The daemon may also cancel. It does this by writing a special word
"GUESTFS_CANCEL_FLAG" to the socket. The library listens for this
during the transfer, and if it gets it, it will cancel the transfer (it sends
a cancel chunk). The special word is chosen so that even if cancellation
happens right at the end of the transfer (after the library has finished
writing and has started listening for the reply), the "spurious"
cancel flag will not be confused with the reply message.
This protocol allows the transfer of arbitrary sized files (no 32 bit limit),
and also files where the size is not known in advance (eg. from pipes or
sockets). However the chunks are rather small
("GUESTFS_MAX_CHUNK_SIZE"), so that neither the library nor the
daemon need to keep much in memory.
FUNCTIONS THAT HAVE FILEOUT PARAMETERS
The protocol for FileOut parameters is exactly the same as for FileIn
parameters, but with the roles of daemon and library reversed.
total length (header + ret,
but not including the length word itself,
and not including the chunks)
struct guestfs_message_header (encoded as XDR)
struct guestfs_<foo>_ret (encoded as XDR)
sequence of chunks for FileOut param #0
sequence of chunks for FileOut param #1 etc.
初期メッセージ
When the daemon launches it sends an initial word
("GUESTFS_LAUNCH_FLAG") which indicates that the guest and daemon is
alive. This is what "guestfs_launch" in
guestfs(3) waits for.
PROGRESS NOTIFICATION MESSAGES
The daemon may send progress notification messages at any time. These are
distinguished by the normal length word being replaced by
"GUESTFS_PROGRESS_FLAG", followed by a fixed size progress message.
The library turns them into progress callbacks (see
"GUESTFS_EVENT_PROGRESS" in
guestfs(3)) if there is a
callback registered, or discards them if not.
The daemon self-limits the frequency of progress messages it sends (see
"daemon/proto.c:notify_progress"). Not all calls generate progress
messages.
When libguestfs (or libguestfs tools) are run, they search a path looking for an
appliance. The path is built into libguestfs, or can be set using the
"LIBGUESTFS_PATH" environment variable.
Normally a supermin appliance is located on this path (see "SUPERMIN
APPLIANCE" in
supermin(1)). libguestfs reconstructs this into a
full appliance by running "supermin --build".
However, a simpler "fixed appliance" can also be used. libguestfs
detects this by looking for a directory on the path containing all the
following files:
- •
- kernel
- •
- initrd
- •
- root
- •
-
README.fixed (note that it must be present as
well)
If the fixed appliance is found, libguestfs skips supermin entirely and just
runs the virtual machine (using qemu or the current backend, see
"BACKEND" in
guestfs(3)) with the kernel, initrd and root
disk from the fixed appliance.
Thus the fixed appliance can be used when a platform or a Linux distribution
does not support supermin. You build the fixed appliance on a platform that
does support supermin using
libguestfs-make-fixed-appliance(1), copy it
over, and use that to run libguestfs.
guestfs(3),
guestfs-hacking(1),
guestfs-examples(3),
libguestfs-test-tool(1),
libguestfs-make-fixed-appliance(1),
http://libguestfs.org/.
Richard W.M. Jones ("rjones at redhat dot com")
Copyright (C) 2009-2020 Red Hat Inc.
To get a list of bugs against libguestfs, use this link:
https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools
To report a new bug against libguestfs, use this link:
https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools
When reporting a bug, please supply:
- •
- The version of libguestfs.
- •
- Where you got libguestfs (eg. which Linux distro, compiled
from source, etc)
- •
- Describe the bug accurately and give a way to reproduce
it.
- •
- Run libguestfs-test-tool(1) and paste the
complete, unedited output into the bug report.