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

[Stefan Hajnoczi]

On Mon, Dec 05, 2016 at 04:21:22PM +0100, Greg Kurz 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 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.

Hi Greg,
I won't merge this for QEMU 2.8 because this 2.7 issue is not a 2.8
release blocker and it's too risky (good points have been raised in this
thread).

Please target -stable when consensus has been reached.

Thanks,
Stefan

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