Re: [PATCH v4 8/8] migration: Introduce POSTCOPY_DEVICE state

[Peter Xu <peterx@redhat.com>]

On Mon, Nov 03, 2025 at 07:32:57PM +0100, Juraj Marcin wrote:
> From: Juraj Marcin <jmarcin@redhat.com>
> 
> Currently, when postcopy starts, the source VM starts switchover and
> sends a package containing the state of all non-postcopiable devices.
> When the destination loads this package, the switchover is complete and
> the destination VM starts. However, if the device state load fails or
> the destination side crashes, the source side is already in
> POSTCOPY_ACTIVE state and cannot be recovered, even when it has the most
> up-to-date machine state as the destination has not yet started.
> 
> This patch introduces a new POSTCOPY_DEVICE state which is active while
> the destination machine is loading the device state, is not yet running,
> and the source side can be resumed in case of a migration failure.
> Return-path is required for this state to function, otherwise it will be
> skipped in favor of POSTCOPY_ACTIVE.
> 
> To transition from POSTCOPY_DEVICE to POSTCOPY_ACTIVE, the source
> side uses a PONG message that is a response to a PING message processed
> just before the POSTCOPY_RUN command that starts the destination VM.
> Thus, this feature is effective even if the destination side does not
> yet support this new state.
> 
> Signed-off-by: Juraj Marcin <jmarcin@redhat.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu