Re: [Qemu-devel] [PATCH for-2.8] qdev: apply global properties in reverse order

[Eduardo Habkost]

On Mon, Dec 05, 2016 at 06:25:55PM +0100, Cornelia Huck wrote:
> On Mon, 5 Dec 2016 14:48:29 -0200
> Eduardo Habkost <address@hidden> wrote:
> 
> > On Mon, Dec 05, 2016 at 04:42:00PM +0100, Cornelia Huck wrote:
> > > On Mon, 05 Dec 2016 16:21:22 +0100
> > > Greg Kurz <address@hidden> wrote:
> > > 
> > > > The current code recursively applies global properties from child up to
> > > > parent. So, if you have:
> > > > 
> > > > -global virtio-pci.disable-modern=on
> > > > -global virtio-blk-pci.disable-modern=off
> > > > 
> > > > Then the default value of disable-modern for a virtio-blk-pci device is 
> > > > on,
> > > > which looks wrong from an OOP perspective.
> > > > 
> > > > This patch reverses the logic, so that a child property always prevail.
> > > 
> > > This sounds reasonable...
> > > 
> > > > 
> > > > This fixes a subtle bug that got introduced in 2.7 with commit 
> > > > "9a4c0e220d8a
> > > > hw/virtio-pci: fix virtio behaviour" for older (< 2.7) machine types: 
> > > > the
> > > > HW_COMPAT_2_6 macro contains global virtio-pci.disable-* properties 
> > > > which
> > > > would silently override global properties passed on the command line for
> > > > virtio subtypes.
> > > > 
> > > > Signed-off-by: Greg Kurz <address@hidden>
> > > > ---
> > > > 
> > > > AFAIK, libvirt's XML doesn't know about modern/legacy modes for virtio
> > > > devices. Early adopters of virtio 1.0 had to rely on the 
> > > > <qemu:commandline>
> > > > tag to pass global properties to QEMU. This patch ensures that XML files
> > > > used with older machine types remain valid with newer versions of QEMU.
> > > > 
> > > > FWIW I guess it could help to have this fix in 2.8, and also probably in
> > > > 2.7.1.
> > > 
> > > ...but I'm a bit worried about doing that change this late in the
> > > cycle, as we may introduce subtle changes for other configurations. At
> > > the very least, we should look over the existing backwards compat
> > > properties (I'll look at those I'm familiar with).
> > 
> > This patch would change the behavior for:
> >  -global virtio-blk-pci.disable-modern=on
> >  -global virtio-pci.disable-modern=off
> > 
> > And I am not sure the new behavior would be correct. Shouldn't we
> > apply the properties in the order specified in the command-line?
> 
> Probably; but how should this interact with compat props?

compat props should be always applied in the order they appear.
-global should always be applied after compat props.

So, it looks like we have two additional reasons to just follow
the order the global properties were registered.

> 
> > 
> > On either case, changing the semantics of the command-line can
> > break existing configurations. Let's do it more carefully in the
> > 2.9 cycle, and fix the existing bug by changing the HW_COMPAT_*
> > macros?
> 
> Changing the compat props is probably the best option at this point in
> time. Let's take this slowly so we can come up with a reasonable
> solution for 2.9.

Agreed.

-- 
Eduardo