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

[Peter Maydell]

On Thu, 25 Jul 2019 at 14:43, Paolo Bonzini <address@hidden> wrote:
> To me *maintainability is the biggest consideration* when introducing a
> new feature.  "We can do just as well with q35" is a good reason to
> deprecate and delete microvm, but not a good reason to reject it now as
> long as microvm is good enough in terms of maintainability.

I think maintainability matters, but also important is "are
we going in the right direction in the first place?".
virtio-mmio is (variously deliberately and accidentally)
quite a long way behind virtio-pci, and certain kinds of things
(hotplug, extensibility beyond a certain number of endpoints)
are not going to be possible (either ever, or without a lot
of extra design and implementation work to reimplement stuff
we have already today with PCI). Are we sure we're not going
to end up with a stream of "oh, now we need to implement X for
virtio-mmio (that virtio-pci already has)", "users want Y now
(that virtio-pci already has)", etc?

The other thing is that once we've introduced something we're
stuck with whatever it does, because we don't like breaking
backwards compatibility. So I think getting the virtio-legacy
vs virtio-1 story sorted out before we land microvm is
important, at least to the point where we know we haven't
backed ourselves into a corner or required a lot of extra
effort on transitional-device support that we could have
avoided.

Which isn't to say that I'm against the microvm approach;
just that I'd like us to consider and make a decision on
these issues before landing it, rather than just saying
"the patches in themselves look good, let's merge it".

thanks
-- PMM