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

[Greg Kurz <groug@kaod.org>]

On Fri, 9 Dec 2016 18:06:50 -0200
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Wed, Dec 07, 2016 at 02:39:25PM +0100, Greg Kurz wrote:
> > On Tue, 6 Dec 2016 17:31:59 -0200
> > Eduardo Habkost <ehabkost@redhat.com> 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 <groug@kaod.org>    
> > > > 
> > > > 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:
> > > >   
> [...]
> > > > 
> > > > 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. :)  
> 
> Now my question is: what should we do in 2.8.1?
> 
> 1) Change the rules and apply "[PATCH] qdev: fix the order compat
>    and global properties are applied"

Would it be more acceptable to change the rules in 2.8.1 than it is
for 2.8 ?

> 2) Fix only virtio-pci (apply a new version of "[PATCH] virtio:
>    fix HW_COMPAT_2_6 macro for virtio-*-pci drivers")

The patch is huge, but we're sure that it doesn't break anything else.

> 3) Apply this hack so we fix all compat_props cases without
>    changing ordering rules. (And then change the ordering rules
>    in 2.9)
> 

FWIW, I think this patch is ok, and not that hacky: it simply implements the
idea that 'compat properties of abstract classes are actually handled at the
non-abstract sub-classes level'. Of course, it would be great to document this
behavior somewhere. :)

Cheers.

--
Greg