Re: [Qemu-devel] [PATCH v2 21/21] q35: add acpi-based pci hotplug.

[Gerd Hoffmann]

  Hi,

>>> I just tried out getting rid of the bridges by default.
>>
>> That clearly raises the question which devices should be created
>> automatically by -M q35.  I think the devices which are part of the ich9
>> chipset should be there by default.  /me looks at my laptop which
>> happens to have a ich9 chipset.
> 
> The reason this is a bad idea is very simple: we only have a way to add
> devices not to remove them.  So if you miss a device which your guest
> needs, it is easy to add, but there is no way to remove.

Why would you want remove devices?  They don't harm when present.  And
you can't remove them on real hardware either.  Try ordering a ich9
without sound or usb ;)

>> Real ich9 has the e1000 @ 00:19.0, so it would make sense to place one
>> there.  Adding a default nic will probably create some headache though,
>> so maybe better don't.
> 
> And that's just one example.

It is problematic because the nic needs configuration and configuring a
builtin device is tricky.  Also our e1000 model isn't the ich9 one.

Most other chipset devices are not problematic at all as they are just
controllers where you can attach stuff to (and by default there isn't
anything attached).  This includes:

  * pcie ports (waiting for pcie devices plugged in).
  * ahci controller (waiting for disks/cdroms being attached).
  * intel-hda (waiting for audio coded (hda-*) being attached).
  * ehci+uhci (waiting for usb devices being plugged in).

The stuff being attached/plugged there needs configuration, but not the
controllers themself.

> Fact is, we don't emulate real hardware exactly.

But we try to.

> So let's have a minimal machine and if you want to add e1000 audio etc,
> you can do this. We can even teach management to do it with friendly
> UI as opposed to cryptic machine types.

/me disagrees.

cheers,
  Gerd