On Mon, Mar 13, 2023 at 06:48:32PM +0100, Hanna Czenczek wrote:
Add a virtio-fs-specific vhost-user interface to facilitate migrating
back-end-internal state. We plan to migrate the internal state simply
Luckily the interface does not need to be virtiofs-specific since it
only transfers opaque data. Any stateful device can use this for
migration. Please make it generic both at the vhost-user protocol
message level and at the QEMU vhost API level.
as a binary blob after the streaming phase, so all we need is a way to
transfer such a blob from and to the back-end. We do so by using a
dedicated area of shared memory through which the blob is transferred in
chunks.
Keeping the migration data transfer separate from the vhost-user UNIX
domain socket is a good idea since the amount of data could be large and
may congest the UNIX domain socket. The shared memory interface solves
this.
Where I get lost is why it needs to be shared memory instead of simply
an fd? On the source, the front-end could read the fd until EOF and
transfer the opaque data. On the destination, the front-end could write
to the fd and then close it. I think that would be simpler than the
shared memory interface and could potentially support zero-copy via
splice(2) (QEMU doesn't need to look at the data being transferred!).
Here is an outline of an fd-based interface:
- SET_DEVICE_STATE_FD: The front-end passes a file descriptor for
transferring device state.
The @direction argument:
- SAVE: the back-end transfers an outgoing device state over the fd.
- LOAD: the back-end transfers an incoming device state over the fd.
The @phase argument:
- STOPPED: the device is stopped.
- PRE_COPY: reserved for future use.
- POST_COPY: reserved for future use.
The back-end transfers data over the fd according to @direction and
@phase upon receiving the SET_DEVICE_STATE_FD message.
There are loose ends like how the message interacts with the virtqueue
enabled state, what happens if multiple SET_DEVICE_STATE_FD messages are
sent, etc. I have ignored them for now.
What I wanted to mention about the fd-based interface is:
- It's just one message. The I/O activity happens via the fd and does
not involve GET_STATE/SET_STATE messages over the vhost-user domain
socket.
- Buffer management is up to the front-end and back-end implementations
and a bit simpler than the shared memory interface.
Did you choose the shared memory approach because it has certain
advantages?
This patch adds the following vhost operations (and implements them for
vhost-user):
- FS_SET_STATE_FD: The front-end passes a dedicated shared memory area
to the back-end. This area will be used to transfer state via the
other two operations.
(After the transfer FS_SET_STATE_FD detaches the shared memory area
again.)
- FS_GET_STATE: The front-end asks the back-end to place a chunk of
internal state into the shared memory area.
- FS_SET_STATE: The front-end puts a chunk of internal state into the
shared memory area, and asks the back-end to fetch it.
On the source side, the back-end is expected to serialize its internal
state either when FS_SET_STATE_FD is invoked, or when FS_GET_STATE is
invoked the first time. On subsequent FS_GET_STATE calls, it memcpy()s
parts of that serialized state into the shared memory area.
On the destination side, the back-end is expected to collect the state
blob over all FS_SET_STATE calls, and then deserialize and apply it once
FS_SET_STATE_FD detaches the shared memory area.
What is the rationale for waiting to receive the entire incoming state
before parsing it rather than parsing it in a streaming fashion? Can
this be left as an implementation detail of the vhost-user back-end so
that there's freedom in choosing either approach?