Add a virtio-fs-specific vhost-user interface to facilitate migrating
back-end-internal state. We plan to migrate the internal state simply
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.
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.
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
include/hw/virtio/vhost-backend.h | 9 ++
include/hw/virtio/vhost.h | 68 +++++++++++++++
hw/virtio/vhost-user.c | 138 ++++++++++++++++++++++++++++++
hw/virtio/vhost.c | 29 +++++++
4 files changed, 244 insertions(+)