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