Re: [PATCH V4 5/8] migration: cpr-exec save and load

["Cédric Le Goater" <clg@redhat.com>]

On 9/22/25 15:49, Steve Sistare wrote:
> 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.

Couldn't we preserve some memory to hand off to QEMU ? Like firmwares
An environment variable is a limited method.

Thanks,

C.



That's a short term hack right ? it's not even documented. I am sure
you something else in mind.

> These new functions are called in a subsequent patch.
> 
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> ---
>   include/migration/cpr.h |  5 +++
>   migration/cpr-exec.c    | 94 +++++++++++++++++++++++++++++++++++++++++++++++++
>   migration/meson.build   |  1 +
>   3 files changed, 100 insertions(+)
>   create mode 100644 migration/cpr-exec.c
> 
> diff --git a/include/migration/cpr.h b/include/migration/cpr.h
> index 2b074d7..b84389f 100644
> --- a/include/migration/cpr.h
> +++ b/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
> diff --git a/migration/cpr-exec.c b/migration/cpr-exec.c
> new file mode 100644
> index 0000000..2c32e9c
> --- /dev/null
> +++ b/migration/cpr-exec.c
> @@ -0,0 +1,94 @@
> +/*
> + * 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 = memfd_create(CPR_EXEC_STATE_NAME, 0);

The build should be adjusted for Linux only.

Thanks,

C.



> +
> +    if (mfd < 0) {
> +        error_setg_errno(errp, errno, "memfd_create failed");
> +        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);
> +}
> diff --git a/migration/meson.build b/migration/meson.build
> index 0f71544..16909d5 100644
> --- a/migration/meson.build
> +++ b/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',