Re: [Qemu-devel] [PATCHv6 3/5] fw_cfg: move assert() and linking of fw_cfg device to the machine into instance_init()

[Eduardo Habkost]

On Fri, Jun 23, 2017 at 09:12:01AM +0100, Mark Cave-Ayland wrote:
> On 21/06/17 14:23, Eduardo Habkost wrote:
> 
> >>>>> I now have a v7 patchset ready to go (currently hosted at
> >>>>> https://github.com/mcayland/qemu/tree/fwcfg7 for the curious). Laszlo,
> >>>>> I've currently left off your Tested-by tag since I'm not sure it's still
> >>>>> valid for less-than-trivial changes - if you're happy for me to re-add
> >>>>> it before I send the v7 patchset to the list, please let me know.
> >>>>
> >>>> I intend to test v7 when you post it.
> >>>
> >>> I still see the instance_init assert() in that branch (commit
> >>> 17d75643f880).  Is that correct?
> >>
> >> Yes that was the intention. In 17d75643f880 both the assert() and
> >> object_property_add_child() are moved into the instance_init() function,
> >> and then in the follow-up commit eddedb5 the assert() is removed from
> >> instance_init() and the object_resolve_path_type() check added into
> >> fw_cfg_init1() as part of its conversion into the
> >> fw_cfg_common_realize() function.
> > 
> > We can't move assert() + linking to instance_init even if it's
> > just temporary, as it makes device-list-properties crash.
> > 
> > Just linking in instance_init is also a problem, because
> > instance_init should never affect global QEMU state.
> > 
> > You could move linking to realize as well, but: just like you
> > already moved sysbus_add_io() calls outside realize, I believe
> > linking belongs outside realize too.  I need to re-read the
> > discussion threads to understand the motivation behind that.
> 
> Ultimately the question we're trying to answer is "has someone
> instantiated another fw_cfg device for this machine?" and the way it
> works currently is that fw_cfg_init_io() and fw_cfg_init_mem() attach
> the fw_cfg device to the /machine object and then check after realize
> whether a /machine/fw_cfg device already exists, aborting if it does.
> 
> So in the current implementation we're not actually concerned with the
> placement of fw_cfg within the model itself, and indeed with a fixed
> location in the QOM tree it's already completely wrong. If you take a
> look at the QOM tree for the sparc/sparc64/ppc machines you'll see that
> they really are very far from reality.
> 
> For me the use of object_resolve_path_type() during realize is a good
> solution since regardless of the location of the fw_cfg we can always
> detect whether we have more than one device instance.
> 
> However what seems unappealing about this is that while all existing
> users which use fw_cfg_init_io() and fw_cfg_init_mem() are fine, in the
> case where I am wiring up the device myself then for my sun4u example
> the code looks like this:
> 
> dev = qdev_create(NULL, TYPE_FW_CFG_IO);
> ...
> qdev_init_nofail(dev);
> memory_region_add_subregion(pci_address_space_io(ebus), BIOS_CFG_IOPORT,
>                             &FW_CFG_IO(dev)->comb_iomem);
> 
> Here you can see that the device is active because it is mapped into the
> correct IO address space, but notice I have forgotten to link it to the
> QOM /machine object myself. Hence I can instantiate and map as many
> instances as I like and never trigger the duplicate instance check which
> makes the check fairly ineffective.

This is a good point, but I have a question about that: will something
break if a machine decides to create two fw_cfg objects and map them to
different addresses?  If it won't break, I see no reason to try to
enforce a single instance in the device code.  If it will break, then a
check in realize is still a hack, but might be a good enough solution.

-- 
Eduardo