Re: [Qemu-devel] [PATCH 17/33] virtio-pci: add flags to enable/disable legacy/modern

[Gerd Hoffmann <kraxel@redhat.com>]

On Do, 2015-06-04 at 11:52 +0100, Daniel P. Berrange wrote:
> On Thu, Jun 04, 2015 at 12:34:26PM +0200, Gerd Hoffmann wrote:
> > Add VIRTIO_PCI_FLAG_DISABLE_LEGACY and VIRTIO_PCI_FLAG_DISABLE_MODERN
> > for VirtIOPCIProxy->flags.  Also add properties for them.  They can be
> > used to disable modern (virtio 1.0) or legacy (virtio 0.9) modes.
> > 
> > By default only legacy is advertized, modern will be turned on by
> > default once all remaining spec compilance issues are addressed.
> 
> What is the reason for adding these flags? From the pov of management
> app integration I think we'd really want the front/back to be automatically
> negotiating the right compatible featureset. Having mgmt need to set these
> legacy/modern flags based on guest OS needs seems rather painful. As such
> I'd be really loathe to support these in libvirt

Usually there should be no need to muck with these, except for
development and testing purposes.  And of course we need them so we can
turn off virtio 1.0 for old machine types via compat properties (once we
flip the switch to enable virtio-1.0 by default).

libvirt support for these in the domain xml syntax goes into
"nice-to-have" category, but I can equally well live without them and
use the <qemu:*> tags to set them if needed.

Support for them higher up in the management stack should not be needed.

cheers,
  Gerd