Re: [PATCH v2 06/29] migration: Add auto-pause capability

[Fabiano Rosas <farosas@suse.de>]

Markus Armbruster <armbru@redhat.com> writes:

> Fabiano Rosas <farosas@suse.de> writes:
>
>> Add a capability that allows the management layer to delegate to QEMU
>> the decision of whether to pause a VM and perform a non-live
>> migration. Depending on the type of migration being performed, this
>> could bring performance benefits.
>>
>> Note that the capability is enabled by default but at this moment no
>> migration scheme is making use of it.
>
> This sounds as if the capability has no effect unless the "migration
> scheme" (whatever that may be) opts into using it.  Am I confused?
>

What I mean here is that this capability is implemented and functional,
but I'm not retroactively enabling any existing migration code to use
auto-pause. Otherwise people would start seeing their guests being
paused before migraton in scenarios they never used to pause.

By "migration scheme" I mean types of migration. Or modes of
operation. Or exclusive parameters. Anything that is different enough
from what exists today that we would consider a different type of
migration. Anything that allow us to break backward compatibility
(because it never existed before to begin with).

E.g. this series introduces the fixed-ram migration. That never existed
before. So from the moment we enable that code to use this capability,
it will always do auto-pause, unless the management layer wants to avoid
it.

>> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>
> [...]
>
>> diff --git a/qapi/migration.json b/qapi/migration.json
>> index db3df12d6c..74f12adc0e 100644
>> --- a/qapi/migration.json
>> +++ b/qapi/migration.json
>> @@ -523,6 +523,10 @@
>>  #     and can result in more stable read performance.  Requires KVM
>>  #     with accelerator property "dirty-ring-size" set.  (Since 8.1)
>>  #
>> +# @auto-pause: If enabled, allows QEMU to decide whether to pause the
>> +#     VM before migration for an optimal migration performance.
>> +#     Enabled by default. (since 8.1)
>
> If this needs an opt-in to take effect, it should be documented.
>

Someting like this perhaps?

# @auto-pause: If enabled, allows QEMU to decide whether to pause the VM
#     before migration for an optimal migration performance. Enabled by
#     default. New migration code needs to opt-in at
#     migration_should_pause(), otherwise this behaves as if
#     disabled. (since 8.2)

>> +#
>>  # Features:
>>  #
>>  # @unstable: Members @x-colo and @x-ignore-shared are experimental.
>> @@ -539,7 +543,7 @@
>>             { 'name': 'x-ignore-shared', 'features': [ 'unstable' ] },
>>             'validate-uuid', 'background-snapshot',
>>             'zero-copy-send', 'postcopy-preempt', 'switchover-ack',
>> -           'dirty-limit'] }
>> +           'dirty-limit', 'auto-pause'] }
>>  
>>  ##
>>  # @MigrationCapabilityStatus: