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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 03, 2019 at 12:04:00AM +0200, Sergio Lopez wrote:
> On Tue, Jul 02, 2019 at 07:04:15PM +0100, Peter Maydell wrote:
> > 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
> 
> 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.

OK so please start with adding virtio 1 support. Guest bits
have been ready for years now.

-- 
MST