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

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

On 9/22/2025 12:00 PM, Cédric Le Goater wrote:
> 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.

There is no upside in making this more complicated.  We only need to
pass one tidbit of information -- the file descriptor number of the memfd
that contains all other information.
> Thanks,
> 
> C.
> 
> That's a short term hack right ? it's not even documented. 

It is an implementation detail, known only to the matched saving
and loading functions inside qemu.  No one else needs to know, so
no documentation.

- Steve

>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',
>