Re: [PATCH v2 00/16] Crazy shit around -global (pardon my french)

[Markus Armbruster]

Cc: David for insurance against me spewing nonsense about migration.

John Snow <jsnow@redhat.com> writes:

> On 6/25/20 12:45 AM, Markus Armbruster wrote:
>> John Snow <jsnow@redhat.com> writes:
>> 
>>> On 6/22/20 5:42 AM, Markus Armbruster wrote:
>>>> There are three ways to configure backends:
>>>>
>>>> * -nic, -serial, -drive, ... (onboard devices)
>>>>
>>>> * Set the property with -device, or, if you feel masochistic, with
>>>>   -set device (pluggable devices)
>>>>
>>>> * Set the property with -global (both)
>>>>
>>>> The trouble is -global is terrible.
>>>>
>>>> It gets applied in object_new(), which can't fail.  We treat failure
>>>> to apply -global as fatal error, except when hot-plugging, where we
>>>> treat it as warning *boggle*.  I'm not addressing that today.
>>>>
>>>> Some code falls apart when you use both -global and the other way.
>>>>
>>>> To make life more interesting, we gave -drive two roles: with
>>>> interface type other than none, it's for configuring onboard devices,
>>>> and with interface type none, it's for defining backends for use with
>>>> -device and such.  Since we neglect to require interface type none for
>>>> the latter, you can use one -drive in both roles.  This confuses the
>>>> code about as much as you, dear reader, probably are by now.
>>>>
>>>> Because this still isn't interesting enough, there's yet another way
>>>> to configure backends, just for floppies: set the floppy controller's
>>>> property.  Goes back to the time when floppy wasn't a separate device,
>>>> and involves some Bad Magic.  Now -global can interact with itself!
>>>>
>>>> Digging through all this took me an embarrassing amount of time.
>>>> Hair, too.
>>>>
>>>> My patches reject some the silliest uses outright, and deprecate some
>>>> not so silly ones that have replacements.
>>>>
>>>> Apply on top of my "[PATCH v2 00/58] qdev: Rework how we plug into the
>>>> parent bus".
>>>>
>>>
>>> Oof. Thank you for your work in fixing our darkest corners. I sincerely
>>> appreciate it.
>>>
>>> The qdev tree ordering problems don't cause any issues for migration, do
>>> they?
>> 
>> This series should only change device configuration, not device state or
>> its encoding in the migration stream.
>> 
>> I'm not sure what you mean by "qdev tree ordering problems".  Ist it
>> commit e8c9e65816 'qom: Make "info qom-tree" show children sorted'?
>> 
>>> (I see you already sent a PR, so whatever!)
>> 
>> A question that might avoid a later migration debugging session is
>> *never* "whatever"!
>> 
>
> I thought I had read that one of these patches changes the order in
> which devices get instantiated, which I thought might change their QOM
> paths. Which I thought *might* have some ramifications for migration,
> but wasn't sure.

Device instantiation order changes should not break migration.

The order in which devices appear in the migration stream should not
matter.

> If it's just showing the same path outputs *sorted*, then there's no
> problem.
>
> Likely misread.
>
> --js