On Mon, 5 Dec 2016 15:41:30 -0200 Eduardo Habkost <ehabk...@redhat.com> wrote:
> On Mon, Dec 05, 2016 at 06:25:55PM +0100, Cornelia Huck wrote: > > On Mon, 5 Dec 2016 14:48:29 -0200 > > Eduardo Habkost <ehabk...@redhat.com> 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 <gr...@kaod.org> 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 <gr...@kaod.org> > > > > > --- > > > > > > > > > > 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. > This is actually the way they're being registered to the global_props static list: compat props as they appear in HW_COMPAT_* and then -global as they appear on the command line. > So, it looks like we have two additional reasons to just follow > the order the global properties were registered. > Thinking again, maybe we just need to reverse the logic in another way: go through global_props and apply the property if the device can be casted to the corresponding class (i.e. object_class_dynamic_cast() != NULL). I'll try that. > > > > > > > > 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. >
