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

[Eduardo Habkost]

On Thu, Jan 10, 2019 at 03:01:25PM +0000, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (address@hidden) 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 <address@hidden> 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 <address@hidden>
> > > > > Fixes: f6e501a28ef9 ("virtio: Provide version-specific variants of 
> > > > > virtio PCI devices")
> > > > > Signed-off-by: Eduardo Habkost <address@hidden>
> > > > > ---
> > > > >  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?

-- 
Eduardo