[Virtio-fs] [PATCH v4 5/8] vhost-user.rst: Migrating back-end-internal state

[Stefan Hajnoczi]

On Wed, Oct 04, 2023 at 02:59:01PM +0200, Hanna Czenczek wrote:
> For vhost-user devices, qemu can migrate the virtio state, but not the
> back-end's internal state.  To do so, we need to be able to transfer
> this internal state between front-end (qemu) and back-end.
> 
> At this point, this new feature is added for the purpose of virtio-fs
> migration.  Because virtiofsd's internal state will not be too large, we
> believe it is best to transfer it as a single binary blob after the
> streaming phase.
> 
> These are the additions to the protocol:
> - New vhost-user protocol feature VHOST_USER_PROTOCOL_F_DEVICE_STATE
> - SET_DEVICE_STATE_FD function: Front-end and back-end negotiate a file
>   descriptor over which to transfer the state.
> - CHECK_DEVICE_STATE: After the state has been transferred through the
>   file descriptor, the front-end invokes this function to verify
>   success.  There is no in-band way (through the file descriptor) to
>   indicate failure, so we need to check explicitly.
> 
> Once the transfer FD has been established via SET_DEVICE_STATE_FD
> (which includes establishing the direction of transfer and migration
> phase), the sending side writes its data into it, and the reading side
> reads it until it sees an EOF.  Then, the front-end will check for
> success via CHECK_DEVICE_STATE, which on the destination side includes
> checking for integrity (i.e. errors during deserialization).
> 
> Signed-off-by: Hanna Czenczek <hreitz at redhat.com>
> ---
>  docs/interop/vhost-user.rst | 172 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 172 insertions(+)

Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
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