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

[Alexander Graf <agraf@suse.de>]

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<agraf@suse.de>   wrote:
>>>>> On 29.06.2011, at 15:59, Wei Liu<liuw@liuw.name>   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?


Alex