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

[Peter Maydell <peter.maydell@linaro.org>]

On Tue, 2 Jul 2019 at 18:34, Sergio Lopez <slp@redhat.com> wrote:
> Peter Maydell <peter.maydell@linaro.org> 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

thanks
-- PMM