Re: [Qemu-devel] Should we auto-generate IDs? (was: [PATCH] qdev-monitor.c: Add device id generation)

[Daniel P. Berrange]

On Tue, Aug 25, 2015 at 02:38:17PM +0200, Markus Armbruster wrote:
> You're proposing to revise a qdev design decision, namely the purpose of
> IDs.  This has been discussed before, and IDs remained unchanged.
> Perhaps it's time to revisit this issue.  Cc'ing a few more people.
> 
> Relevant prior threads:
> * [PATCH] qdev: Reject duplicate and anti-social device IDs
>   http://thread.gmane.org/gmane.comp.emulators.qemu/71230/focus=72272
> * [PATCH 6/6] qdev: Generate IDs for anonymous devices
>   http://thread.gmane.org/gmane.comp.emulators.qemu/114853/focus=114858
> * [PATCH] qdev: Assign a default device ID when none is provided.
>   http://thread.gmane.org/gmane.comp.emulators.qemu/249702
> * IDs in QOM (was: [PATCH] util: Emancipate id_wellformed() from QemuOpt
>   http://thread.gmane.org/gmane.comp.emulators.qemu/299945/focus=300381
> 
> Probably more I can't remember anymore :)
> 
> Programmingkid <address@hidden> writes:
> 
> > Add device ID generation to each device if an ID isn't given.
> >
> > Signed-off-by: John Arbuckle <address@hidden>
> >
> > ---
> > This patch can be tested by adding adding usb devices using the monitor.
> > Start QEMU with the -usb option. Then go to the monitor and type
> > "device_add usb-mouse". The ID of the device will be set to a number.
> > Since QEMU will not allow an user to add a device with an ID set to a
> > number, there is no chance for ID collisions. 
> 
> The second sentence should really be part of your commit message.
> The first sentence wouldn't hurt, either.
> 
> Another useful addition would be *why* you want generated IDs.  I
> believe you do because you need them for device_del.
> 
> In prior discussion, we always concluded that device_del should accept
> QOM paths.  It still doesn't.
> 
> Many things in QEMU have IDs.  They all work pretty much the same:
> 
> 1. The ID is set by the user.  If the user doesn't, there is none.
> 
>    Exception: a few old interfaces set well-known IDs.  If the user uses
>    these interfaces, he needs to take care that his own IDs don't clash.
> 
>    Example: drive_add picks an ID based on interface type, media type,
>    bus and unit number.  blockdev_add doesn't.  Instead, it requires the
>    user to pick one.
> 
> 2. The ID must be well-formed.
> 
>    Exception: inconsistently enforced for QOM, see last thread quoted
>    above.
> 
> 3. If the user may need to address the thing, either the ID must be
>    mandatory, or there has to be another way to address it.
> 
>    Example: netdev-add requires ID.  Rationale: the only way to put it
>    to use is referencing it from a device, and that requires an ID.
> 
>    Example: device_add doesn't require ID.  If you don't specify one,
>    you can't device_del it.  Annoying trap for the unwary.  There are
>    *two* other ways to address it: qdev path and QOM path.  qdev path is
>    basically too botched to be usable.  QOM path should do just fine,
>    but device_del doesn't accept it.  It could.
> 
> We could revise rule 1 to always generate IDs, in a way that can't clash
> with the user's IDs (impossible unless rule 2 is actually observed).
> Rule 3 then becomes moot.

If QEMU auto-generates IDs, then the user still has to query QEMU to
figure out what ID was assigned. If the device was not assigned an
ID, then it surely becomes hard for the user to identify which device
they just added in order to ask what its ID is. Which is a chicken
and egg problem. Even if the user could figure out what device they
just added, why go to the extra trouble of querying QEMU to find out
the auto-generated ID, when you could just provide an ID explicitly
upfront avoiding the entire problem. IMHO auto-generating IDs is a
just road to nowhere. Ideally we would mandate user provided IDs
but we sadly can't for back-compat reasons.

Regards,
Daniel
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