migration: cpr-exec save and load

To preserve CPR state across exec, create a QEMUFile based on a memfd, and
keep the memfd open across exec.  Save the value of the memfd in an
environment variable so post-exec QEMU can find it.

These new functions are called in a subsequent patch.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Link: https://lore.kernel.org/r/1759332851-370353-6-git-send-email-steven.sistare@oracle.com
[peterx: fix build for Windows]
Signed-off-by: Peter Xu <peterx@redhat.com>

include/migration/cpr.h

@@ -53,4 +53,9 @@ int cpr_get_fd_param(const char *name, const char *fdname, int index,
 QEMUFile *cpr_transfer_output(MigrationChannel *channel, Error **errp);
 QEMUFile *cpr_transfer_input(MigrationChannel *channel, Error **errp);
 
+QEMUFile *cpr_exec_output(Error **errp);
+QEMUFile *cpr_exec_input(Error **errp);
+void cpr_exec_persist_state(QEMUFile *f);
+bool cpr_exec_has_state(void);
+void cpr_exec_unpersist_state(void);
 #endif

migration/cpr-exec.c

@@ -0,0 +1,99 @@
+/*
+ * Copyright (c) 2021-2025 Oracle and/or its affiliates.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/cutils.h"
+#include "qemu/memfd.h"
+#include "qapi/error.h"
+#include "io/channel-file.h"
+#include "io/channel-socket.h"
+#include "migration/cpr.h"
+#include "migration/qemu-file.h"
+#include "migration/misc.h"
+#include "migration/vmstate.h"
+#include "system/runstate.h"
+
+#define CPR_EXEC_STATE_NAME "QEMU_CPR_EXEC_STATE"
+
+static QEMUFile *qemu_file_new_fd_input(int fd, const char *name)
+{
+    g_autoptr(QIOChannelFile) fioc = qio_channel_file_new_fd(fd);
+    QIOChannel *ioc = QIO_CHANNEL(fioc);
+    qio_channel_set_name(ioc, name);
+    return qemu_file_new_input(ioc);
+}
+
+static QEMUFile *qemu_file_new_fd_output(int fd, const char *name)
+{
+    g_autoptr(QIOChannelFile) fioc = qio_channel_file_new_fd(fd);
+    QIOChannel *ioc = QIO_CHANNEL(fioc);
+    qio_channel_set_name(ioc, name);
+    return qemu_file_new_output(ioc);
+}
+
+void cpr_exec_persist_state(QEMUFile *f)
+{
+    QIOChannelFile *fioc = QIO_CHANNEL_FILE(qemu_file_get_ioc(f));
+    int mfd = dup(fioc->fd);
+    char val[16];
+
+    /* Remember mfd in environment for post-exec load */
+    qemu_clear_cloexec(mfd);
+    snprintf(val, sizeof(val), "%d", mfd);
+    g_setenv(CPR_EXEC_STATE_NAME, val, 1);
+}
+
+static int cpr_exec_find_state(void)
+{
+    const char *val = g_getenv(CPR_EXEC_STATE_NAME);
+    int mfd;
+
+    assert(val);
+    g_unsetenv(CPR_EXEC_STATE_NAME);
+    assert(!qemu_strtoi(val, NULL, 10, &mfd));
+    return mfd;
+}
+
+bool cpr_exec_has_state(void)
+{
+    return g_getenv(CPR_EXEC_STATE_NAME) != NULL;
+}
+
+void cpr_exec_unpersist_state(void)
+{
+    int mfd;
+    const char *val = g_getenv(CPR_EXEC_STATE_NAME);
+
+    g_unsetenv(CPR_EXEC_STATE_NAME);
+    assert(val);
+    assert(!qemu_strtoi(val, NULL, 10, &mfd));
+    close(mfd);
+}
+
+QEMUFile *cpr_exec_output(Error **errp)
+{
+    int mfd;
+
+#ifdef CONFIG_LINUX
+    mfd = qemu_memfd_create(CPR_EXEC_STATE_NAME, 0, false, 0, 0, errp);
+#else
+    mfd = -1;
+#endif
+
+    if (mfd < 0) {
+        return NULL;
+    }
+
+    return qemu_file_new_fd_output(mfd, CPR_EXEC_STATE_NAME);
+}
+
+QEMUFile *cpr_exec_input(Error **errp)
+{
+    int mfd = cpr_exec_find_state();
+
+    lseek(mfd, 0, SEEK_SET);
+    return qemu_file_new_fd_input(mfd, CPR_EXEC_STATE_NAME);
+}

migration/meson.build

@@ -16,6 +16,7 @@ system_ss.add(files(
   'channel-block.c',
   'cpr.c',
   'cpr-transfer.c',
+  'cpr-exec.c',
   'cpu-throttle.c',
   'dirtyrate.c',
   'exec.c',