migration: cpr_get_fd_param helper

Add the helper function cpr_get_fd_param, to use when preserving
a file descriptor that is opened externally and passed to QEMU.
cpr_get_fd_param returns a descriptor number either from a QEMU
command-line parameter, from a getfd command, or from CPR state.

When a descriptor is passed to new QEMU via SCM_RIGHTS, its number
changes.  Hence, during CPR, the command-line parameter is ignored
in new QEMU, and over-ridden by the value found in CPR state.

Similarly, if the descriptor was originally specified by a getfd
command in old QEMU, the fd number is not known outside of QEMU,
and it changes when sent to new QEMU via SCM_RIGHTS.  Hence the
user cannot send getfd to new QEMU, but when the user sends a
hotplug command that references the fd, cpr_get_fd_param finds
its value in CPR state.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/qemu-devel/1751493538-202042-5-git-send-email-steven.sistare@oracle.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/migration/cpr.h b/include/migration/cpr.h
index 07858e9..eb27a93 100644
--- a/include/migration/cpr.h
+++ b/include/migration/cpr.h
@@ -32,6 +32,8 @@ void cpr_state_close(void);
 struct QIOChannel *cpr_state_ioc(void);
 
 bool cpr_incoming_needed(void *opaque);
+int cpr_get_fd_param(const char *name, const char *fdname, int index,
+                     Error **errp);
 
 QEMUFile *cpr_transfer_output(MigrationChannel *channel, Error **errp);
 QEMUFile *cpr_transfer_input(MigrationChannel *channel, Error **errp);