Re: [Qemu-devel] [PATCH] virtio-bus: Plug devices after features are negotiated

[Paolo Bonzini]

On 10/09/2016 10:23, Maxime Coquelin wrote:
> Currently, devices are plugged before features are negotiated.
> If the backend doesn't support VIRTIO_F_VERSION_1, the transport
> need to rewind some settings.
> 
> This is the case for CCW, for which a post_plugged callback had
> been introduced, where max_rev field is just updated if
> VIRTIO_F_VERSION_1 is not supported by the backend.
> For PCI, implementing the post_plugged would be much more
> complicated, so it needs to know whether the backend supports
> VIRTIO_F_VERSION_1 at plug time.
> 
> Currently, nothing is done for PCI. Modern capabilitities get
> exposed to the guest even if VIRTIO_F_VERSION_1 is not supported
> by the backend, which confuses the guest.
> 
> This patch proposes to replace existing post_plugged solution
> with an approach that fits with both transports.
> Features negociation is performed before ->device_plugged() call.
> A pre_plugged callback is introduced so that the transports can
> set their supported features.

Have you tested virtio-mmio too?

Paolo