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

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Jun 23, 2017 at 05:52:03PM +0200, Laszlo Ersek wrote:
> On 06/23/17 13:50, Eduardo Habkost wrote:
> > 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.
> > 
> 
> (1) For the guest, it makes no sense to encounter two fw_cfg devices.
> Also, a lot of the existent code in QEMU assumes at most one fw_cfg
> device (for example, there is some related ACPI generation).

This is an argument for making board code ensure there's only one
device, and possibly for providing a helper that board code can use.
But it doesn't require validation on realize.

> 
> (2) Relatedly, the fw_cfg_find() helper function is used quite widely,
> and it shouldn't break -- either due to more-than-one device instances,
> or due to the one fw_cfg device being linked under a path that is
> different from FW_CFG_PATH.

This is also an argument for providing a helper that ensures
fw_cfg_find() will work, but doesn't require validation on realize.


All that said, I don't have a strong argument against doing it on
realize, except my gut feeling that this is not how qdev was
designed[1].  If doing it on realize is the simplest way to do it, I
won't be the one complaining.

[1] I believe the original intent was to make every single device
    user-creatable and define boards in a declarative way in config
    files.  We are very far from that goal.

-- 
Eduardo