Re: [Qemu-devel] Questions on DeviceState and Virtio infrastructure

[Stefano Stabellini]

On Thu, 30 Jun 2011, Alexander Graf wrote:
> On 06/30/2011 12:21 PM, Stefano Stabellini wrote:
> > On Thu, 30 Jun 2011, Wei Liu wrote:
> >> On Thu, Jun 30, 2011 at 4:31 PM, Alexander Graf<address@hidden>  wrote:
> >>> On 29.06.2011, at 15:59, Wei Liu<address@hidden>  wrote:
> >>>
> >>>> Hi, QEMU folks
> >>>>
> >>>> I know that I might have a bad title for this post, but I just don't
> >>>> have better idea for the title.
> >>>>
> >>>> I'm developing virtio support for Xen pv guest, hoping to reuse the
> >>>> virtio infrastructure in qemu, i.e. I'm planning to use qemu as
> >>>> "virtio backend" for Xen pv. And qemu can be run as pv backend if
> >>>> proper "machine" option is given.
> >>>>
> >>>> Maybe you've known that Xen pv guest utilizes Xenbus/Xenstore to
> >>>> configure its paravirt devices. So I'm to configure virtio devices
> >>>> with Xenbus/Xenstore as well. But in nowdays XenDevice in qemu does
> >>>> not include a DeviceState. To my understanding, it is not connected to
> >>>> qemu's internal buses or whatever (correct me if I'm wrong, I'm
> >>>> relatively new to qemu).
> >>> I'm not a huge fan of adding multiple different transports for virtio if 
> >>> we don't have to. IIRC Xen PV guests can do PCI device assignment, right? 
> >>> That means there is a PCI bus for them which we could reuse to run 
> >>> virtio-pci on.
> >>>
> >>> By then, you'd get all the virtio code for free and don't have to worry 
> >>> about maintaining yet another transport (which _is_ cumbersome)
> >>>
> >> Good point, I will check this.
> >>
> >> I thought that Xen pv pci assignment is only used for pci passthrough.
> >> But I will check the possibility to assign pv backend to guest.
> >>
> >> Stefano, how would you say about this?
> >
> > Yes, the PCI bus is an empty PCI bus created by xen-pcifront and it is
> > only used for PCI passthrough. It couldn't be used as is for virtio-pci.
> 
> What's keeping us from doing so? Doesn't MMIO emulation work with Xen PV?

No, it does not.
The only realistic way that I can see to make virtio work with PV guests
is to write "virtio-xenstore".