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migration: cpr_get_fd_param helper

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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>
This commit is contained in:
Steve Sistare 2025-07-02 14:58:41 -07:00 • committed by Cédric Le Goater
parent 7ed0919119
commit ccfc6715cf
2 changed files with 39 additions and 0 deletions
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2
include/migration/cpr.h
View file

@ -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);

37
migration/cpr.c
View file

@ -13,6 +13,7 @@
#include "migration/qemu-file.h"
#include "migration/savevm.h"
#include "migration/vmstate.h"
#include "monitor/monitor.h"
#include "system/runstate.h"
#include "trace.h"
@ -264,3 +265,39 @@ bool cpr_incoming_needed(void *opaque)
MigMode mode = migrate_mode();
return mode == MIG_MODE_CPR_TRANSFER;
}
/*
* cpr_get_fd_param: find a descriptor and return its value.
*
* @name: CPR name for the descriptor
* @fdname: An integer-valued string, or a name passed to a getfd command
* @index: CPR index of the descriptor
* @errp: returned error message
*
* If CPR is not being performed, then use @fdname to find the fd.
* If CPR is being performed, then ignore @fdname, and look for @name
* and @index in CPR state.
*
* On success returns the fd value, else returns -1.
*/
int cpr_get_fd_param(const char *name, const char *fdname, int index,
Error **errp)
{
ERRP_GUARD();
int fd;
if (cpr_is_incoming()) {
fd = cpr_find_fd(name, index);
if (fd < 0) {
error_setg(errp, "cannot find saved value for fd %s", fdname);
}
} else {
fd = monitor_fd_param(monitor_cur(), fdname, errp);
if (fd >= 0) {
cpr_save_fd(name, index, fd);
} else {
error_prepend(errp, "Could not parse object fd %s:", fdname);
}
}
return fd;
}