[PATCH V3 25/42] migration: cpr_get_fd_param helper

[Steve Sistare <steven.sistare@oracle.com>]

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>
---
 include/migration/cpr.h |  2 ++
 migration/cpr.c         | 40 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/include/migration/cpr.h b/include/migration/cpr.h
index 5f1ff10..353fd34 100644
--- a/include/migration/cpr.h
+++ b/include/migration/cpr.h
@@ -32,6 +32,8 @@ struct QIOChannel *cpr_state_ioc(void);
 
 bool cpr_needed_for_reuse(void *opaque);
 void cpr_kvm_close(void);
+int cpr_get_fd_param(const char *name, const char *fdname, int index,
+                     bool *reused, Error **errp);
 
 void cpr_transfer_init(void);
 QEMUFile *cpr_transfer_output(MigrationChannel *channel, Error **errp);
diff --git a/migration/cpr.c b/migration/cpr.c
index 6102d04..179aed1 100644
--- a/migration/cpr.c
+++ b/migration/cpr.c
@@ -14,6 +14,7 @@
 #include "migration/qemu-file.h"
 #include "migration/savevm.h"
 #include "migration/vmstate.h"
+#include "monitor/monitor.h"
 #include "system/kvm.h"
 #include "system/runstate.h"
 #include "trace.h"
@@ -260,3 +261,42 @@ void cpr_kvm_close(void)
     kvm_close();
     vfio_kvm_device_close();
 }
+
+/*
+ * 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
+ * @reused: returns true if the fd is found in CPR state, else false.
+ * @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,
+                     bool *reused, 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);
+        }
+        *reused = true;
+    } 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);
+        }
+        *reused = false;
+    }
+    return fd;
+}
-- 
1.8.3.1