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

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 11 Jan 2019 01:06:45 +0400
Marc-André Lureau <marcandre.lureau@gmail.com> wrote:

> On Thu, Jan 10, 2019 at 10:07 PM Eduardo Habkost <ehabkost@redhat.com> wrote:
> >
> > On Thu, Jan 10, 2019 at 03:01:25PM +0000, Dr. David Alan Gilbert wrote:  
> > > * Michael S. Tsirkin (mst@redhat.com) wrote:  
> > > > 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?  
> > >
> > > Well the word is a separate issue; 'optional' is probably a bad choice
> > > but that's less important than what it actually does.
> > > I'd go for skip_if_missing  which is explicit.  
> >
> > I'm unsure about that.  To me "optional" means "it can be
> > missing" and carries the same information as "skip if missing".
> >
> > What do others think?  
> 
> skip_if_missing is more explicit.

I'd slightly prefer optional, but skip_if_missing is fine.