Re: [PATCH v5 5/6] vhost_user: Make source quit after reporting migration state

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Feb 05, 2025 at 01:39:03AM +0100, Stefano Brivio wrote:
> On migration, the source process asks passt-helper to set TCP sockets
> in repair mode, dumps the information we need to migrate connections,
> and closes them.
> 
> At this point, we can't pass them back to passt-helper using
> SCM_RIGHTS, because they are closed, from that perspective, and
> sendmsg() will give us EBADF. But if we don't clear repair mode, the
> port they are bound to will not be available for binding in the
> target.
> 
> Terminate once we're done with the migration and we reported the
> state. This is equivalent to clearing repair mode on the sockets we
> just closed.

As we've discussed, quitting still makes sense, but the description
above is not really accurate.  Perhaps,

===
Once we've passed the migration's "point of no return", there's no way
to resume the guest on the source side, because we no longer own the
connections.  There's not really anything we can do except exit.
===

Except.. thinking about it, I'm not sure that's technically true.
After migration, the source qemu enters a kind of limbo state.  I
suppose for the case of to-disk migration (savevm) the guest can
actually be resumed.  Which for us is not really compatible with
completing at least a local migration properly.  Not really sure what
to do about that.

I think it's also technically possible to use monitor commands to boot
up essentially an entirely new guest instance in the original qemu,
in which case for us it would make sense to basically reset ourselves
(flush the low table).

Hrm.. we really need to know the sequence of events in a bit more
detail to get this right (not that this stops improving the guts of
the logic in the meantime).

I'm asking around to see if I can find who did the migration stuff or
virtiofsd, so we can compare notes.

> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>  vhost_user.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/vhost_user.c b/vhost_user.c
> index b107d0f..70773d6 100644
> --- a/vhost_user.c
> +++ b/vhost_user.c
> @@ -997,6 +997,8 @@ static bool vu_send_rarp_exec(struct vu_dev *vdev,
>  	return false;
>  }
>  
> +static bool quit_on_device_state = false;
> +
>  /**
>   * vu_set_device_state_fd_exec() - Set the device state migration channel
>   * @vdev:	vhost-user device
> @@ -1024,6 +1026,9 @@ static bool vu_set_device_state_fd_exec(struct vu_dev *vdev,
>  	migrate_request(vdev->context, msg->fds[0],
>  			direction == VHOST_USER_TRANSFER_STATE_DIRECTION_LOAD);
>  
> +	if (direction == VHOST_USER_TRANSFER_STATE_DIRECTION_SAVE)
> +		quit_on_device_state = true;
> +
>  	/* We don't provide a new fd for the data transfer */
>  	vmsg_set_reply_u64(msg, VHOST_USER_VRING_NOFD_MASK);
>  
> @@ -1201,4 +1206,10 @@ void vu_control_handler(struct vu_dev *vdev, int fd, uint32_t events)
>  
>  	if (reply_requested)
>  		vu_send_reply(fd, &msg);
> +
> +	if (quit_on_device_state &&
> +	    msg.hdr.request == VHOST_USER_CHECK_DEVICE_STATE) {
> +		info("Migration complete, exiting");
> +		exit(EXIT_SUCCESS);
> +	}
>  }

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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