[libvirt] [Qemu-devel] clean/simple Q35 support in libvirt+QEMU for guest OSes that don't support virtio-1.0

[Markus Armbruster]

Eduardo Habkost <ehabkost at redhat.com> writes:

> On Thu, Aug 23, 2018 at 05:26:47PM +0100, Daniel P. Berrangé wrote:
> [...]
>> There are countless mistakes in both QEMU & libvirt, but only some of
>> them are worth the cost of changing.

Agreed.

>>                                      I'm not seeing a compelling reason
>> why this change is worthwhile. The impact of the design mistake is narrow
>> and only raised because of downstream desire to change even legacy OS
>> to use Q35 when there's no benefit to those OS of such a change.
>
> I think you underestimate the impact of the design mistake.

And overstate the "this is just for a downstream need".

> Maintaining and working around badly designed interfaces have
> costs.
>
> The virtio device model was already an obstacle when designing
> new bus/device introspection interfaces.  It will be an obstacle
> for adding mechanisms to tell applications that legacy virtio
> devices can't be plugged on PCI Express slots.

Thus, there's a genuine upstream motivation to clean up this mess.
Whether it's worthwhile is of course a fair question.

The argument for "it is worthwhile" I like to see in general is patches.

> Anyway, if we want to fix the design mistake it wouldn't make
> sense to do it only on the libvirt side and not on QEMU.  We can
> address that on QEMU first, and then let libvirt decide how to
> handle it.

Yes.