Re: [PATCH V1 4/4] cpr: reboot mode

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

On 10/20/2023 5:45 AM, Juan Quintela wrote:
> Steve Sistare <steven.sistare@oracle.com> wrote:
>> Add the cpr-reboot migration mode.  Usage:
>>
>> $ qemu-system-$arch -monitor stdio ...
>> QEMU 8.1.50 monitor - type 'help' for more information
>> (qemu) migrate_set_capability x-ignore-shared on
>> (qemu) migrate_set_parameter mode cpr-reboot
>> (qemu) migrate -d file:vm.state
>> (qemu) info status
>> VM status: paused (postmigrate)
>> (qemu) quit
>>
>> $ qemu-system-$arch -monitor stdio -incoming defer ...
>> QEMU 8.1.50 monitor - type 'help' for more information
>> (qemu) migrate_set_capability x-ignore-shared on
>> (qemu) migrate_set_parameter mode cpr-reboot
>> (qemu) migrate_incoming file:vm.state
>> (qemu) info status
>> VM status: running
>>
>> In this mode, the migrate command saves state to a file, allowing one
>> to quit qemu, reboot to an updated kernel, and restart an updated version
>> of qemu.  The caller must specify a migration URI that writes to and reads
>> from a file.  Unlike normal mode, the use of certain local storage options
>> does not block the migration, but the caller must not modify guest block
>> devices between the quit and restart.  The guest RAM memory-backend must
>> be shared, and the @x-ignore-shared migration capability must be set,
>> to avoid saving RAM to the file.  Guest RAM must be non-volatile across
>> reboot, such as by backing it with a dax device, but this is not enforced.
>> The restarted qemu arguments must match those used to initially start qemu,
>> plus the -incoming option.
> 
> Please, add this message to doc/<somewhere> instead (or additionally) to
> the commit log.
> 
>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
>> ---
>>  qapi/migration.json | 16 +++++++++++++++-
>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/qapi/migration.json b/qapi/migration.json
>> index 184fb78..2d862fa 100644
>> --- a/qapi/migration.json
>> +++ b/qapi/migration.json
>> @@ -620,9 +620,23 @@
>>  #
>>  # @normal: the original form of migration. (since 8.2)
>>  #
>> +# @cpr-reboot: The migrate command saves state to a file, allowing one to
>> +#              quit qemu, reboot to an updated kernel, and restart an updated
>> +#              version of qemu.  The caller must specify a migration URI
>> +#              that writes to and reads from a file.  Unlike normal mode,
>> +#              the use of certain local storage options does not block the
>> +#              migration, but the caller must not modify guest block devices
>> +#              between the quit and restart.  The guest RAM memory-backend
>> +#              must be shared, and the @x-ignore-shared migration capability
>> +#              must be set, to avoid saving it to the file.  Guest RAM must
>> +#              be non-volatile across reboot, such as by backing it with
>> +#              a dax device, but this is not enforced.  The restarted qemu
>> +#              arguments must match those used to initially start qemu, plus
>> +#              the -incoming option. (since 8.2)
>> +#
>>  ##
>>  { 'enum': 'MigMode',
>> -  'data': [ 'normal' ] }
>> +  'data': [ 'normal', 'cpr-reboot' ] }
>>  
>>  ##
>>  # @BitmapMigrationBitmapAliasTransform:
> 
> It only works with file backend, and we don't have any check for that.
> Wondering how to add that check.

Actually, it works for other backends, but the ram contents are saved in the
state file, which is slower. I should spell that out in the json comment and
in the commit message.

> Additionally, you are not adding a migration test that does exactly what
> you put there in the comment.

I provide tests/avocado/cpr.py in the original long series.  Would you
like me to add it to this series, or post it later?  Would you prefer I
add a test to tests/qtest/migration-test.c?

- Steve