Re: [Qemu-devel] [SeaBIOS] [PATCH] don't expose pvpanic device in the UI

[Markus Armbruster <armbru@redhat.com>]

Gleb Natapov <gleb@redhat.com> writes:

> On Tue, Aug 06, 2013 at 01:03:34PM +0200, Andreas Färber wrote:
>> Am 06.08.2013 12:44, schrieb Gleb Natapov:
>> > On Tue, Aug 06, 2013 at 01:19:53PM +0300, Michael S. Tsirkin wrote:
>> >>>> It's a QEMU issue, devices that are added with -device are
>> >>>> documented in -device help and removed by dropping them from
>> >>>> command line. Devices added by default have no way to
>> >>>> be dropped from QOM except -nodefaults.
>> >>>>
>> >>> Are you saying that because pvpanic is added automatically QEMU -device
>> >>> help does not print help about it? Why not fix that? What QEMU --help
>> >>> issues has to do with deciding which devices should or should not be
>> >>> present by default?
>> >>
>> >> No, I'm saying what I said: that there's no way to remove a device
>> >> added by default except -nodefaults, and no way to
>> >> find out what does -nodefaults exclude so you
>> >> can add things you need back selectively.
>> >>
>> > And what are the rules that govern device exclusion from -nodefaults
>> > list? Why -nodefaults does not create empty machine?
>> 
>> We have -M none to create an empty machine.
>> 
>> FWIW -M q35 does not create all Q35 devices, there's -readconfig
>> docs/q35-chipset.cfg for the rest. The criteria certainly is not
>> migratability, since ICH9 AHCI (part of -M q35) is unmigratable,
>> unfortunately.
>> One practical reason not to create everything via config is that we
>> cannot create SysBusDevices via -device when they require MMIO mapping
>> or IRQ setup.

Support wiring up a machine without board code, just configuration has
been the ever-distant goal of the qdev effort.

>>               For ISADevices such as pvpanic that's not a problem.
>> Anthony has proposed QOM'ifying MemoryRegions and qemu_irq as solution
>> to do the wiring-up from command line or config file, but those attempts
>> got stuck a long time ago.
>> 
> But -M creates not only things that cannot be created from a command
> line, it includes some default set of devices, so what is the criteria
> for those?

I'm not aware of defined, coherent criteria.

I can give you descriptive rather than prescriptive, though.  Used to be
"whatever anyone felt users would want".  It's now "whatever has always
been there, plus whatever survives interminable bikeshedding^W^Wvigorous
debate.