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

[Markus Armbruster]

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"!