Re: [Qemu-devel] [PATCHv7 5/6] fw_cfg: move qdev_init_nofail() from fw_cfg_init1() to callers

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Jul 07, 2017 at 04:58:17PM +0200, Igor Mammedov wrote:
> On Fri, 7 Jul 2017 10:13:00 -0300
> "Eduardo Habkost" <ehabkost@redhat.com> wrote:
[...]
> > I don't disagree with adding the assert(), but it looks like
> > making fw_cfg_find() return NULL if there are multiple devices
> > can be useful for realize.
> > 
> > In this case, it looks like Mark is relying on that in
> > fw_cfg_common_realize(): if multiple devices are created,
> > fw_cfg_find() will return NULL, and realize will fail.  This
> > sounds like a more graceful way to handle multiple-device
> > creation than crashing on fw_cfg_find().  This is the solution
> > used by find_vmgenid_dev()/vmgenid_realize(), BTW.
> 
> I suspect that find_vmgenid_dev() works by luck as it could be
> placed only as /machine/peripheral-anon/foo1 or /machine/peripheral/foo2
>   object_resolve_partial_path() : machine
>            object_resolve_partial_path() : peripheral-anon => foo1
>            object_resolve_partial_path() : peripheral => foo2
>            if (found /* foo2 */) {
>                if (obj /* foo1 */) {
>                    return NULL;

I don't think this is luck: object_resolve_partial_path() is
explicitly documented to always return NULL if multiple matches
are found, and I don't see any bug in its implementation that
would break that functionality.

-- 
Eduardo