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

[Stefano Stabellini]

On Thu, 30 Jun 2011, Alexander Graf wrote:
> On 06/30/2011 04:11 PM, Stefano Stabellini wrote:
> > 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".
> 
> 
> Hrm. What's technically keeping you from doing MMIO with PV guests?

All the infrastructure to catch MMIO reads/writes in Xen and forward
them to Qemu is not enabled; Qemu MMIO emulation is not running.
At that point is just better to run a PV on HVM guest.