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

[John Snow]

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.

If it's just showing the same path outputs *sorted*, then there's no
problem.

Likely misread.

--js