Re: [Qemu-devel] [RFC] virtio-mmio: implement modern (v2) personality (virtio-1)

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jul 31, 2019 at 02:22:09PM +0200, Sergio Lopez wrote:
> 
> Michael S. Tsirkin <mst@redhat.com> writes:
> 
> > On Mon, Jul 29, 2019 at 02:57:55PM +0200, Sergio Lopez wrote:
> >> Implement the modern (v2) personality, according to the VirtIO 1.0
> >> specification.
> >> 
> >> Support for v2 among guests is not as widespread as it'd be
> >> desirable. While the Linux driver has had it for a while, support is
> >> missing, at least, from Tianocore EDK II, NetBSD and FreeBSD.
> >
> > The fact that there was no open source hypervisor implementation has
> > probably contributed to this :)
> >
> >> For this reason, the v2 personality is disabled, keeping the legacy
> >> behavior as default.
> >
> > I agree it's a good default for existing machine types.
> >
> >> Machine types willing to use v2, can enable it
> >> using MachineClass's compat_props.
> >
> > Hmm. Are compat_props really the recommended mechanism to
> > tweak defaults? I was under the impression it's
> > only for compatibility with old machine types.
> > Eduardo, any opinion on this?
> 
> Stefan suggested using something like "-global virtio-mmio.modern=true"
> which does the trick for the command line, but I'd also like a way to
> set it to true by default on microvm. We can discuss the best way to
> achieve that (if using compat_props isn't acceptable) on the next
> microvm patch series.

Compatibility is the most common use case, but IMO compat_props
can be used for other kinds of machine-specific defaults.  It's
better than burying defaults inside non-introspectable machine
initialization functions.

-- 
Eduardo