[PATCH V1 06/26] migration: precreate vmstate for exec

[Steve Sistare]

Provide migration_precreate_save for saving precreate vmstate across exec.
Create a memfd, save its value in the environment, and serialize state
to it.  Reverse the process in migration_precreate_load.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
---
 include/migration/misc.h |   5 ++
 migration/meson.build    |   1 +
 migration/precreate.c    | 139 +++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 145 insertions(+)
 create mode 100644 migration/precreate.c

diff --git a/include/migration/misc.h b/include/migration/misc.h
index c9e200f..cf30351 100644
--- a/include/migration/misc.h
+++ b/include/migration/misc.h
@@ -56,6 +56,11 @@ AnnounceParameters *migrate_announce_params(void);
 
 void dump_vmstate_json_to_file(FILE *out_fp);
 
+/* migration/precreate.c */
+int migration_precreate_save(Error **errp);
+void migration_precreate_unsave(void);
+int migration_precreate_load(Error **errp);
+
 /* migration/migration.c */
 void migration_object_init(void);
 void migration_shutdown(void);
diff --git a/migration/meson.build b/migration/meson.build
index f76b1ba..50e7cb2 100644
--- a/migration/meson.build
+++ b/migration/meson.build
@@ -26,6 +26,7 @@ system_ss.add(files(
   'ram-compress.c',
   'options.c',
   'postcopy-ram.c',
+  'precreate.c',
   'savevm.c',
   'socket.c',
   'tls.c',
diff --git a/migration/precreate.c b/migration/precreate.c
new file mode 100644
index 0000000..0bf5e1f
--- /dev/null
+++ b/migration/precreate.c
@@ -0,0 +1,139 @@
+/*
+ * Copyright (c) 2022, 2024 Oracle and/or its affiliates.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/cutils.h"
+#include "qemu/memfd.h"
+#include "qapi/error.h"
+#include "io/channel-file.h"
+#include "migration/misc.h"
+#include "migration/qemu-file.h"
+#include "migration/savevm.h"
+
+#define PRECREATE_STATE_NAME "QEMU_PRECREATE_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);
+}
+
+static int memfd_create_named(const char *name, Error **errp)
+{
+    int mfd;
+    char val[16];
+
+    mfd = memfd_create(name, 0);
+    if (mfd < 0) {
+        error_setg_errno(errp, errno, "memfd_create failed");
+        return -1;
+    }
+
+    /* Remember mfd in environment for post-exec load */
+    qemu_clear_cloexec(mfd);
+    snprintf(val, sizeof(val), "%d", mfd);
+    g_setenv(name, val, 1);
+
+    return mfd;
+}
+
+static int memfd_find_named(const char *name, int *mfd_p, Error **errp)
+{
+    const char *val = g_getenv(name);
+
+    if (!val) {
+        *mfd_p = -1;
+        return 0;       /* No memfd was created, not an error */
+    }
+    g_unsetenv(name);
+    if (qemu_strtoi(val, NULL, 10, mfd_p)) {
+        error_setg(errp, "Bad %s env value %s", PRECREATE_STATE_NAME, val);
+        return -1;
+    }
+    lseek(*mfd_p, 0, SEEK_SET);
+    return 0;
+}
+
+static void memfd_delete_named(const char *name)
+{
+    int mfd;
+    const char *val = g_getenv(name);
+
+    if (val) {
+        g_unsetenv(name);
+        if (!qemu_strtoi(val, NULL, 10, &mfd)) {
+            close(mfd);
+        }
+    }
+}
+
+static QEMUFile *qemu_file_new_memfd_output(const char *name, Error **errp)
+{
+    int mfd = memfd_create_named(name, errp);
+
+    if (mfd < 0) {
+        return NULL;
+    }
+
+    return qemu_file_new_fd_output(mfd, name);
+}
+
+static QEMUFile *qemu_file_new_memfd_input(const char *name, Error **errp)
+{
+    int ret, mfd;
+
+    ret = memfd_find_named(name, &mfd, errp);
+    if (ret || mfd < 0) {
+        return NULL;
+    }
+
+    return qemu_file_new_fd_input(mfd, name);
+}
+
+int migration_precreate_save(Error **errp)
+{
+    QEMUFile *f = qemu_file_new_memfd_output(PRECREATE_STATE_NAME, errp);
+
+    if (!f) {
+        return -1;
+    } else if (qemu_savevm_precreate_save(f, errp)) {
+        memfd_delete_named(PRECREATE_STATE_NAME);
+        return -1;
+    } else {
+        /* Do not close f, as mfd must remain open. */
+        return 0;
+    }
+}
+
+void migration_precreate_unsave(void)
+{
+    memfd_delete_named(PRECREATE_STATE_NAME);
+}
+
+int migration_precreate_load(Error **errp)
+{
+    int ret;
+    QEMUFile *f = qemu_file_new_memfd_input(PRECREATE_STATE_NAME, errp);
+
+    if (!f) {
+        return -1;
+    }
+    ret = qemu_savevm_precreate_load(f, errp);
+    qemu_fclose(f);
+    g_unsetenv(PRECREATE_STATE_NAME);
+    return ret;
+}
-- 
1.8.3.1