Re: [Qemu-devel] [PATCH 0/2] Pointer properties and device_add

[Markus Armbruster]

Andreas Färber <address@hidden> writes:

> Am 01.12.2013 14:13, schrieb Marcel Apfelbaum:
>> On Fri, 2013-11-29 at 10:43 +0100, address@hidden wrote:
>>> From: Markus Armbruster <address@hidden>
>>>
>>> Pointer properties can be set only by code, not by device_add.  A
>>> device with a pointer property can't work with device_add only unless
>>> the property may remain null.  cannot_instantiate_with_device_add_yet
>>> needs to be set then.  PATCH 1/2 sets it when needed and else
>>> documents why not.  PATCH 2/2 documents this for future users of
>>> pointer properties.
>>>
>>> This applies on top of my "[PATCH v4 00/10] Clean up and fix no_user"
>>> series.
>> 
>> Even that I am not familiar with this code, I've checked all the changes
>> and I agree with them.
>> 
>> Reviewed-by: Marcel Apfelbaum <address@hidden>
>> 
>> Anyway, I do have a question:
>> Why not asserting on qdev_device_add if we have a pointer property?

This is a really good thought.  In fact, it occurred to me, too.
However, see "unless the property may remain null" above: there are uses
of pointer properties that do *not* make the device unusable with
device_add.  We even have an example: etraxfs,pic; see PATCH 1/1.  It's
a sysbus device, so it's unavailable anyway.  But there certainly could
be a device with an optional property that does not and should not have
cannot_instantiate_with_device_add_yet set.

> When we do device_add / device-add, the guest is usually running and we
> shouldn't kill a running guest just because the user is trying something
> stupid that we can easily prevent. ;)

You have a point on assert(bad_input), but this would be
assert(programming_error), where the error is "device doesn't have
cannot_instantiate_with_device_add_yet set".  I'm advocating to be
ruthless with programming error asserts.

> The alternative BTW is dropping all those pointer properties and
> replacing them with link<> properties. Paolo tried that for the OMAP
> timers once but I fear that series was never picked up...?

/* FIXME: Remove opaque pointer properties.  */

/* Not a proper property, just for dirty hacks.  TODO Remove it!  */

:)

>> Instead of checking only cannot_instantiate_with_device_add_yet,
>> we can go over properties and if we have a pointer property, assert or
>> return...
>
> Raising an error for certain property types may be an option. Although
> theoretically the existence of an incompatible property would not
> necessarily indicate incompatibility to instantiate the device, in
> practice I believe we don't have such excess properties.

We don't have them now.  I hope we won't permit any new pointer
properties.  If you guys want pointer property imply its owner's
cannot_instantiate_with_device_add_yet, even though it's not generally
necessary, I'm fine with that.

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