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>

2 files changed, 39 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);
diff --git a/migration/cpr.c b/migration/cpr.c
index a50a57e..535d587 100644
--- a/migration/cpr.c
+++ b/migration/cpr.c
@@ -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;
+}