Re: [Qemu-devel] [RFC for-2.8] machine: Convert abstract typename on compat_props to subclass names

[Greg Kurz]

On Tue, 6 Dec 2016 17:31:59 -0200
Eduardo Habkost <address@hidden> wrote:

> On Tue, Dec 06, 2016 at 05:19:52PM -0200, Eduardo Habkost wrote:
> > On Tue, Dec 06, 2016 at 06:50:47PM +0100, Greg Kurz wrote:  
> > > Since commit "9a4c0e220d8a hw/virtio-pci: fix virtio behaviour", passing
> > > -device virtio-blk-pci.disable-modern=off has no effect on 2.6 machine
> > > types because the internal virtio-pci.disable-modern=on compat property
> > > always prevail.
> > > 
> > > This should ideally be fixed in the qdev properties core code, but it is
> > > too late in the QEMU 2.8 schedule. So this patch fixes the issue by 
> > > setting
> > > the compat properties for every virtio-*-pci subtypes instead of the base
> > > virtio-pci type.
> > > 
> > > Signed-off-by: Greg Kurz <address@hidden>  
> > 
> > So, it looks like the bug is present in many other cases...
> > 
> > I have hacked QEMU to print a warning in case the driver name in
> > compat_props refer to an abstract class or a class that have any
> > subclasses. The results are below:
> > 
> >  apic-common.legacy-instance-id set for abstract class
> >  apic-common.vapic set for abstract class
> >  i386-cpu.arat set for abstract class
> >  i386-cpu.check set for abstract class
> >  i386-cpu.cpuid-0xb set for abstract class
> >  i386-cpu.fill-mtrr-mask set for abstract class
> >  i386-cpu.full-cpuid-auto-level set for abstract class
> >  i386-cpu.l3-cache set for abstract class
> >  i386-cpu.pmu set for abstract class
> >  pci-device.command_serr_enable set for abstract class
> >  pci-device.rombar set for abstract class
> >  pci-device.x-pcie-lnksta-dllla set for abstract class
> >  powerpc64-cpu.pre-2.8-migration set for abstract class
> >  s390-skeys.migration-enabled set for abstract class
> >  spapr-pci-host-bridge.ddw set for superclass
> >  spapr-pci-host-bridge.dynamic-reconfiguration set for superclass
> >  spapr-pci-host-bridge.mem64_win_size set for superclass
> >  spapr-pci-host-bridge.mem_win_size set for superclass
> >  spapr-pci-host-bridge.pre-2.8-migration set for superclass
> >  usb-device.full-path set for abstract class
> >  usb-device.msos-desc set for abstract class
> >  virtio-pci.disable-legacy set for abstract class
> >  virtio-pci.disable-modern set for abstract class
> >  virtio-pci.migrate-extra set for abstract class
> >  virtio-pci.page-per-vq set for abstract class
> >  virtio-pci.virtio-pci-bus-master-bug-migration set for abstract class
> >  virtio-pci.x-disable-pcie set for abstract class
> >  x86_64-cpu.arat set for abstract class
> >  x86_64-cpu.check set for abstract class
> >  x86_64-cpu.cpuid-0xb set for abstract class
> >  x86_64-cpu.fill-mtrr-mask set for abstract class
> >  x86_64-cpu.full-cpuid-auto-level set for abstract class
> >  x86_64-cpu.l3-cache set for abstract class
> >  x86_64-cpu.pmu set for abstract class
> > 
> > I believe the cases where we are likely to cause real-world bugs
> > are virtio-pci and the *-cpu classes (because -cpu is translated
> > to -global).
> > 
> > I'm not sure what should be the right fix in 2.8. I am
> > considering a temporary hack to translate abstract class names in
> > compat_props to global properties for all subclasses, in case
> > they refer to an abstract class. This way we fix the bug where
> > -global doesn't override compat_props properly, but keep the
> > rules for -global untouched.  
> 
> What about this?
> 

Indeed that would have been a clever workaround but it is too late anyway. :)

BTW, that makes me think about Halil's comment in another mail, even if it
is not strictly related to the comapt issue (but rather because I mentioned
it in the changelog of my first patch):

"So I can't even tell if -global virtio-pci.disable-modern=off is even legit
on the command line."

And indeed, -device doesn't accept abstract classes... shouldn't -global
prohibit setting properties for them ?

> (untested)
> 
> Signed-off-by: Eduardo Habkost <address@hidden>
> ---
>  hw/core/machine.c | 35 ++++++++++++++++++++++++++++++++---
>  1 file changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index b0fd91f..49f9007 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -554,6 +554,25 @@ static void machine_class_finalize(ObjectClass *klass, 
> void *data)
>      g_free(mc->name);
>  }
>  
> +static void register_compat_prop(const char *driver,
> +                                 const char *property,
> +                                 const char *value)
> +{
> +    GlobalProperty *p = g_new0(GlobalProperty, 1);
> +    /* Machine compat_props must never cause errors: */
> +    p->errp = &error_abort;
> +    p->driver = driver;
> +    p->property = property;
> +    p->value = value;
> +    qdev_prop_register_global(p);
> +}
> +
> +static void machine_register_compat_for_subclass(ObjectClass *oc, void 
> *opaque)
> +{
> +    GlobalProperty *p = opaque;
> +    register_compat_prop(object_class_get_name(oc), p->property, p->value);
> +}
> +
>  void machine_register_compat_props(MachineState *machine)
>  {
>      MachineClass *mc = MACHINE_GET_CLASS(machine);
> @@ -565,10 +584,20 @@ void machine_register_compat_props(MachineState 
> *machine)
>      }
>  
>      for (i = 0; i < mc->compat_props->len; i++) {
> +        ObjectClass *oc;
>          p = g_array_index(mc->compat_props, GlobalProperty *, i);
> -        /* Machine compat_props must never cause errors: */
> -        p->errp = &error_abort;
> -        qdev_prop_register_global(p);
> +        oc = object_class_by_name(p->driver);
> +        if (oc && object_class_is_abstract(oc)) {
> +            /* temporary hack to make sure we will never override
> +             * globals set explicitly on -global: if an abstract class
> +             * is on compat_props, register globals for each of their
> +             * subclasses instead.
> +             */
> +            object_class_foreach(machine_register_compat_for_subclass,
> +                                 p->driver, false, p);
> +        } else {
> +            register_compat_prop(p->driver, p->property, p->value);
> +        }
>      }
>  }
>