Re: [PATCH v2 7/8] scsi: clean up persistent reservation state after migration

[Eric Blake <eblake@redhat.com>]

On Tue, Oct 13, 2026 at 02:17:51PM -0400, Stefan Hajnoczi wrote:
> Live migration of persistent reservations is designed to work without
> coordination between the source and destination. The destination uses
> the PREEMPT command to atomically take over the reservation or
> unregister keys belonging to the source. This approach avoids race
> conditions where other nodes in the cluster might acquire the
> reservation and it minimizes communication during the downtime window.
> 
> While this works at the SCSI level, this approach is incomplete when
> using multipathd. Multipathd tracks registered keys and automatically
> registers them again when paths come up. It is unaware that the
> destination has preempted the reservation and therefore it will still
> attempt to register the key on the source host.
> 
> Use the new PR_HELPER_CLEANUP command to communicate with multipathd on
> the source after migration completes successfully. The effect is similar
> to REGISTER AND IGNORE EXISTING KEY, except it also discards
> multipathd's state so that the source will no longer attempt to register
> the key.
> 
> PR_HELPER_CLEANUP is a QEMU-internal command that is rejected by
> scsi_req_new(), so note that this command is not exposed to guests
> although direct blk_aio_ioctl(SG_IO) calls can make use of it.
> 
> Buglink: https://redhat.atlassian.net/browse/RHEL-274966
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  include/hw/scsi/scsi.h |  1 +
>  hw/scsi/scsi-disk.c    | 21 ++++++++++++++++++++-
>  hw/scsi/scsi-generic.c | 29 +++++++++++++++++++++++++++++
>  3 files changed, 50 insertions(+), 1 deletion(-)
> 

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org