On Fri, Jun 23, 2017 at 06:48:40PM +0200, Laszlo Ersek wrote: > On 06/23/17 18:10, Eduardo Habkost wrote: > > 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. > > I agree. > > If you read back the discussion threads under the earlier versions of > this patch set, you'll find that I argued for keeping the unicity stuff > -- and possibly some other stuff -- that currently resides in the > fw_cfg_init1() *helper* function outside of device code (realize or > otherwise). > > I didn't disagree with the reorganization of the code, but I suggested > to preserve this stuff in helper functions, which board code could call. > This would clearly place the same burden on Mark's new sun4u board code > -- i.e., that new board code would *also* have to call these helper > function(s). Mark disliked this board requirement (he only wanted to > instantiate the device and be done with it, in board code); and we went > from there. > > Really, please go back to the earlier discussion around fw_cfg_init1() > and you'll see my original point (which matches what you just voiced).
Yep. I was just not sure validation on realize was necessary or convenient. It looks like we agree it is just convenient. > > > 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. > > I'm fine either way, I just wanted to accommodate Mark's preference, > because he seemed to strongly dislike having to call any helper > functions from board code, beyond initing and realizing the device. > > This is my recollection of the earlier discussion anyway. I think we are in agreement, then. If there are no objections from others against doing object_property_add_child() on realize, I'm also OK with that. -- Eduardo
