Re: [PATCH RFC] migration: warn about non-migratable configurations unless '--no-migration' was specified

[Vitaly Kuznetsov <vkuznets@redhat.com>]

"Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:

> * Daniel P. Berrangé (berrange@redhat.com) wrote:
>> On Thu, Apr 15, 2021 at 05:44:02PM +0200, Vitaly Kuznetsov wrote:
>> > When a migration blocker is added nothing is reported to the user,
>> > inability to migrate such guest may come as a late surprise. As a bare
>> > minimum, we can print a warning. To not pollute the output for those, who
>> > have no intention to migrate their guests, introduce '--no-migration'
>> > option which both block the migration and eliminates warning from
>> 
>> I wonder how this is actually going to work in practice ?
>> 
>> At the time libvirt starts a guest, it has no idea whether the guest
>> is likely to need migration 3, 6, 12, 24 months in to the future.
>> 
>> IOW, we can't use a --no-migration flag and will be stuck with these
>> warnings no mtter what.
>> 
>> Is it possible to query the migration blockers via QMP ?
>
> It's possible to query the currently active ones, as of 6.0; from my
> commit  3af8554bd068576b0399087583df48518a2a98f6 it appears in the
> output of query-migrate in the 'blocked-reasons' list.
>
> The HMP equivalent is a64aec725ea0b26fa4e44f8b8b8c72be9aaa4230 showing:
>
>     (qemu) info migrate
>     globals:
>     store-global-state: on
>     only-migratable: off
>     send-configuration: on
>     send-section-footer: on
>     decompress-error-check: on
>     clear-bitmap-shift: 18
>     Outgoing migration blocked:
>       Migration is disabled when VirtFS export path '/home' is mounted in the guest using mount_tag 'fs'
>       non-migratable device: 0000:00:01.2/1/usb-serial
>     

FWIW, this patch makes '--no-migration' an 'ultimate big hammer' so not
matter how many blockers are there, the output will look like:

(qemu) info migrate
globals:
store-global-state: on
only-migratable: off
send-configuration: on
send-section-footer: on
decompress-error-check: on
clear-bitmap-shift: 18
Outgoing migration blocked:
  Guest is not migratable ('--no-migration' used)

-- 
Vitaly