Re: [PATCH 0/9] vhost-user: Migration support

On 19/12/2024 12:13, Laurent Vivier wrote:
...
  This series allows a QEMU guest to be migrated while
it is connected
 to Passt using vhost-user interface.
 
 There are two parts:
 
 - first part enables the migration of QEMU without transferring the
    internal state of Passt. All connections are lost.
 
    This is done by implementing VHOST_USER_SET_LOG_FD and
    VHOST_USER_SET_LOG_BASE commands (and enabling
    VHOST_USER_PROTOCOL_F_LOG_SHMFD feature)
 
    "vhost-user: add VHOST_USER_SET_LOG_FD command"
    "vhost-user: add VHOST_USER_SET_LOG_BASE command"
    "vhost-user: Report to front-end we support
VHOST_USER_PROTOCOL_F_LOG_SHMFD"
 
 - second part allows source Passt instance to send its internal
    state using QEMU migration channel to destination Passt instance.
 
    This is done implementing VHOST_USER_SET_DEVICE_STATE_FD and
    VHOST_USER_CHECK_DEVICE_STATE (and enabling
    VHOST_USER_PROTOCOL_F_DEVICE_STATE feature).
 
    "vhost-user: add VHOST_USER_CHECK_DEVICE_STATE command"
    "vhost-user: add VHOST_USER_SET_DEVICE_STATE_FD command"
    "vhost-user: Report to front-end we support
VHOST_USER_PROTOCOL_F_DEVICE_STATE"
 
    For now, it only implements the function needed to transfer the
    state but no state is transferred.
 
    VHOST_USER_PROTOCOL_F_DEVICE_STATE is implemented in QEMU
    only for vhost-user-fs, to be able to use it with virtio-net
    I have proposed a patch upstream:
 
    https://patchew.org/QEMU/20241218143453.1573185-1-lvivier@redhat.com/
 
 Laurent Vivier (9):
    virtio: Use const pointer for vu_dev
    vhost-user: update protocol features and commands list
    vhost-user: add VHOST_USER_SET_LOG_FD command
    vhost-user: Pass vu_dev to more virtio functions
    vhost-user: add VHOST_USER_SET_LOG_BASE command
    vhost-user: Report to front-end we support
      VHOST_USER_PROTOCOL_F_LOG_SHMFD
    vhost-user: add VHOST_USER_CHECK_DEVICE_STATE command
    vhost-user: add VHOST_USER_SET_DEVICE_STATE_FD command
    vhost-user: Report to front-end we support
      VHOST_USER_PROTOCOL_F_DEVICE_STATE
 
   epoll_type.h |   2 +
   passt.c      |   4 +
   util.h       |   3 +
   vhost_user.c | 251 ++++++++++++++++++++++++++++++++++++++++++++++++++-
   vhost_user.h |  50 +++++++++-
   virtio.c     | 116 +++++++++++++++++++++---
   virtio.h     |  32 +++++--
   vu_common.c  |  59 +++++++++++-
   vu_common.h  |   3 +-
   9 files changed, 484 insertions(+), 36 deletions(-)
  
Another point:

vhost-user can ask backend (passt) to send a notification at the end of the migration on 
the destination side to the network to update the network topology. Do we need this?

This is VHOST_USER_SEND_RARP:
"Ask vhost user back-end to broadcast a fake RARP to notify the migration is
terminated 
for guest that does not support GUEST_ANNOUNCE.

Only legal if feature bit VHOST_USER_F_PROTOCOL_FEATURES is present in 
VHOST_USER_GET_FEATURES and protocol feature bit VHOST_USER_PROTOCOL_F_RARP is present in

VHOST_USER_GET_PROTOCOL_FEATURES. The first 6 bytes of the payload contain the mac address

of the guest to allow the vhost user back-end to construct and broadcast the fake
RARP."

GUEST_ANNOUNCE is a feature that allows vhost-user to ask the kernel to send the 
notification itself at the end of the migration.

Thanks,
Laurent