Re: [Qemu-devel] [PATCH v3 0/4] Introduce the microvm machine type

[Sergio Lopez]

On Tue, Jul 02, 2019 at 07:04:15PM +0100, Peter Maydell wrote:
> On Tue, 2 Jul 2019 at 18:34, Sergio Lopez <address@hidden> wrote:
> > Peter Maydell <address@hidden> writes:
> > > Could we use virtio-pci instead of virtio-mmio? virtio-mmio is
> > > a bit deprecated and tends not to support all the features that
> > > virtio-pci does. It was introduced mostly as a stopgap while we
> > > didn't have pci support in the aarch64 virt machine, and remains
> > > for legacy "we don't like to break existing working setups" rather
> > > than as a recommended config for new systems.
> >
> > Using virtio-pci implies keeping PCI and ACPI support, defeating a
> > significant part of microvm's purpose.
> >
> > What are the issues with the current state of virtio-mmio? Is there a
> > way I can help to improve the situation?
> 
> Off the top of my head:
>  * limitations on numbers of devices
>  * no hotplug support
>  * unlike PCI, it's not probeable, so you have to tell the
>    guest where all the transports are using device tree or
>    some similar mechanism
>  * you need one IRQ line per transport, which restricts how
>    many you can have
>  * it's only virtio-0.9, it doesn't support any of the new
>    virtio-1.0 functionality
>  * it is broadly not really maintained in QEMU (and I think
>    not really in the kernel either? not sure), because we'd
>    rather not have to maintain two mechanisms for doing virtio
>    when virtio-pci is clearly better than virtio-mmio

Some of these are design issues, but others can be improved with a bit
of work.

As for the maintenance burden, I volunteer myself to help with that, so
it won't have an impact on other developers and/or projects.

Sergio.