Re: [Qemu-devel] [PATCH v2 3/3] virtio: Make disable-legacy/disable-modern compat properties optional

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

On Thu, Jan 10, 2019 at 11:31:23AM -0200, Eduardo Habkost wrote:
> On Thu, Jan 10, 2019 at 12:35:26PM +0400, Marc-André Lureau wrote:
> > Hi
> > 
> > On Thu, Jan 10, 2019 at 6:05 AM Eduardo Habkost <ehabkost@redhat.com> wrote:
> > >
> > > The disable-legacy and disable-modern properties apply only to
> > > some virtio-pci devices.  Make those properties optional.
> > >
> > > This fixes the crash introduced by commit f6e501a28ef9 ("virtio: Provide
> > > version-specific variants of virtio PCI devices"):
> > >
> > >   $ qemu-system-x86_64 -machine pc-i440fx-2.6 \
> > >     -device virtio-net-pci-non-transitional
> > >   Unexpected error in object_property_find() at qom/object.c:1092:
> > >   qemu-system-x86_64: -device virtio-net-pci-non-transitional: can't apply \
> > >   global virtio-pci.disable-modern=on: Property '.disable-modern' not found
> > >   Aborted (core dumped)
> > >
> > > Reported-by: Thomas Huth <thuth@redhat.com>
> > > Fixes: f6e501a28ef9 ("virtio: Provide version-specific variants of virtio PCI devices")
> > > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > > ---
> > >  hw/core/machine.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > > index 5530b71981..a19143aa44 100644
> > > --- a/hw/core/machine.c
> > > +++ b/hw/core/machine.c
> > > @@ -91,8 +91,9 @@ const size_t hw_compat_2_7_len = G_N_ELEMENTS(hw_compat_2_7);
> > >
> > >  GlobalProperty hw_compat_2_6[] = {
> > >      { "virtio-mmio", "format_transport_address", "off" },
> > > -    { "virtio-pci", "disable-modern", "on" },
> > > -    { "virtio-pci", "disable-legacy", "off" },
> > > +    /* Optional because not all virtio-pci devices support legacy mode */
> > > +    { "virtio-pci", "disable-modern", "on",  .optional = true },
> > > +    { "virtio-pci", "disable-legacy", "off", .optional = true },
> > 
> > Could the generic devices implement a specific interface instead?
> > virtio-pci-generic?
> 
> This is the kind of complexity I wanted to avoid.  We already
> have too many interface names for subsets of PCI and virtio
> devices.
> 
> > 
> > Adding "optional" handling looks like it may hide some other errors.
> 
> What if we just make "optional" mean "skip if the property
> doesn't exist", as suggested by Cornelia and Dave?

That's the more standard meaning of the word optional, isn't it?

> -- 
> Eduardo