On Fri, Sep 15, 2023 at 12:25:26PM +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 <hre...@redhat.com> > --- > docs/interop/vhost-user.rst | 170 ++++++++++++++++++++++++++++++++++++ > 1 file changed, 170 insertions(+)
Great documentation! Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
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