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

["Cédric Le Goater" <clg@redhat.com>]

On 9/24/25 20:16, Steven Sistare wrote:
> 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.

Please adjust the build for windows, memfd is Linux only.


>> 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.

ok. Fair enough.

Thanks,

C.


> 
> - 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',
>>
>