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

[Laurent Vivier <lvivier@redhat.com>]

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