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

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Aug 22, 2018 at 12:36:27PM +0200, Andrea Bolognani wrote:
> On Tue, 2018-08-21 at 14:21 -0400, Laine Stump wrote:
> > On 08/17/2018 06:35 AM, Andrea Bolognani wrote:
> > > If we decide we want to explicitly spell out the options instead
> > > of relying on QEMU changing behavior based on the slot type, which
> > > is probably a good idea anyway, I think we should have
> > > 
> > >   virtio-0.9 => disable-legacy=no,disable-modern=no
> > >   virtio-1.0 => disable-legacy=yes,disable-modern=no
> > > 
> > > There's basically no reason to have a device legacy-only rather
> > > than transitional, and spelling out both options instead of only
> > > one of them just seems more robust.
> > 
> > I agree with both of those, but the counter-argument is that "virtio"
> > already describes a transitional device like your proposal for
> > virtio-0.9 (at least today), and it makes the versioned models less
> > orthogonal. In the end, I could go either way...
> 
> Yeah, Dan already made that argument and convinced me that we
> should use virtio-0.9 for legacy only, virtio-1.0 for modern only
> and plain virtio for no enforced behavior / transitional.

I don't understand why we are optimizing the new system for the
less useful use cases:

I don't see a use case where virtio-0.9 (legacy-only) would be
more useful than virtio-transitional.  I don't see why anybody
would prefer a legacy-only device instead of a transitional
device.  Even if your guest has only legacy drivers, it might be
upgraded and get new drivers in the future.

I don't see a use case where virtio-1.0 (modern-only) would be
more useful than "virtio".  If you are running i440fx, you get a
transitional device with "virtio", and I don't see why anybody
would prefer a modern-only device.  If you are running Q35, you
already get a modern-only device with "virtio".

The most useful feature users need is the ability to ask for a
transitional virtio device on Q35, and this use case is
explicitly being left out of the proposal.  Why?

-- 
Eduardo