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

[Fabiano Rosas <farosas@suse.de>]

Steve Sistare <steven.sistare@oracle.com> writes:

> 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);

Can we avoid the QEMUFile? I don't see it being exported from this file.

> +
> +    if (!f) {
> +        return -1;
> +    }
> +    ret = qemu_savevm_precreate_load(f, errp);
> +    qemu_fclose(f);
> +    g_unsetenv(PRECREATE_STATE_NAME);
> +    return ret;
> +}