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Re: [Qemu-devel] RFC: -object vs -chardev creation order

From: Markus Armbruster
Subject: Re: [Qemu-devel] RFC: -object vs -chardev creation order
Date: Wed, 18 Mar 2015 10:43:25 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

"Daniel P. Berrange" <address@hidden> writes:

> The current QEMU startup code will create -chardev backends first, then
> create -object backends, then -fsdev backends and so on, in some pretty
> arbitrary order of types.
> There is already a dependancy from the rng-egd object type, which has a
> link to a chardev, which requires -chardev options be processed before
> -object options.
> In my work to introduce an object type for TLS credentials though, we
> encounter exactly the opposite dependancy. A -chardev option will
> require that -object options be processed first, so that the TLS
> credantials can be created.
> This is a no-win situation. Both orders of creation are wrong for some
> set of use cases. This is only going to get worse over time, as more
> types of user creatable objects are defined.
> The application invoking QEMU could easily solve it, by putting the
> args on the command line in the order in which they need to processed,
> but then QEMU throws away this ordering information and creates them
> based on type of backend.

Yes, and that's stupid.

> I'm unclear on whether anyone has thought about ways to solve this yet,
> but it is a blocker for my work now, so I need to figure out some kind
> of solution asap, even if only a temporary workaround.
> One option is to convert the -chardev backends so they become user
> creatable objects, and thus -object can obsolete use of -chardev
> for these. This sounds simple, but I fear it could be a rather large
> yak shaving exercise to get that done.
> Another idea is to perhaps change the way options are stored, so that
> instead of having a separate QemuOptsList for each type of option, have
> one single list and store the type with each entry in the list. Then we
> can iterate over the single list creating things in exactly the same
> order that they were passed on the command line.

It's how we should've done it, but:

>                                                   I fear this may cause
> regressions for apps using QEMU though - eg if some app is placing
> '-object rng-egd' in the args, before the -chardev it would currently
> work, but if we switch to honour ordering it would fail. We could
> introduce a flag -strict-ordering but that feels gross in itself.


I'm afraid we'll have to break our byzantine command line at some
point...  I'd rather not break it now, though.

> A third option is to not process -object args in one go, instead process
> each type of object at a time. eg we'd first create all the
> -object tls-credential instances, then create the -chardev instances,
> then create the -object rng-egd instances. This is probably the least
> amount of work in short term, but not all that scalable, unless we do
> a catch-all default case, so we only need code up hacks for a few
> particular object types.
> Thus my gut feeling is to do option 3, but I'd like other opinions before
> embarking on this....

Piling yet another hack on top isn't great, put telling you "no more
hacks" after everybody and his dog already got to pile on some doesn't
feel fair.

That said: can we sort the creation work topologically?  Requires
identifying references.  In the QemuOpts world, a reference is a
string-valued parameter that is interpreted as ID in a well-known
QemuOptsList.  Adding the necessary information to QemuOptDesc desc[]
shouldn't be too hard.  The ones with empty desc[] could be troublesome,
as usual.

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