Re: [Xen-devel] [PATCH RFC V2 1/5] libxl: add pvusb definitions

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 03/04/2015 10:00 AM, Ian Campbell wrote:
> On Wed, 2015-03-04 at 00:26 -0700, Chun Yan Liu wrote:
>>
>>>>> On 3/3/2015 at 07:10 PM, in message 
>>>>> <1425381019.24959.87.camel@xxxxxxxxxx>, Ian
>> Campbell <ian.campbell@xxxxxxxxxx> wrote: 
>>> On Mon, 2015-01-19 at 16:28 +0800, Chunyan Liu wrote: 
>>>  
>>> Sorry for the long delay in replying. 
>>>  
>>>> To attach a usb device, a virtual usb controller should be created first. 
>>>> This patch defines usbctrl and usbdevice related structs. 
>>>  
>>> Per <54CA17DF0200006600095E3D@xxxxxxxxxxxxxxxxxxxxxxx> please could you 
>>> mention here that the HVM guest related parts (i.e. 
>>> LIBXL_USBCTRL_TYPE_DEVICEMODEL) and libxl_usb_type are placeholders for 
>>> emulated HVM support. 
>>
>> Yes, I agree it's better placed in libxl_usb_type rather than ctrl_type.
>>
>>>  
>>> In fact I wonder if it should just be omitted, we will need a LIBXL_HAVE 
>>> for HVM USB support anyway once it is implemented so we can add the enum 
>>> then. 
>>
>> It won't harm to omit it for current pvusb work. Acceptable to me to
>> add enum later when adding HVM qemu emulated usb device implementation.
> 
> I suppose users of libxl would like to be able to expose to their users
> whether or not HVM USB passthrough will work (i.e. to hide UI options).
> So I think we will want the #define eventually so they can know at
> compile time if HVM USB will work.
> 
> We could add a negative one now (LIBXC_NO_HVM_USB_PASSTHROUGH) and
> remove it later, but that's icky I think.
> 
> So I think omit the HVM stuff for now, it's less confusing overall that
> way.
> 
> George, is that OK with you?

Yes; particularly as I'm hoping that having the PVUSB stuff in will make
it easier for me to add my HVM usb hot-plug stuff before the feature
freeze. :-)

>> To usb controller index.
>> A usb device should be connected to a usb port of a usb controller.
>> e.g.: there is 2 usb controllers in system, each with 8 ports, then:
>> 1st usb controller index will be 0, port will be 1~8.
>> 2nd usb controller index will be 1, port will be 1~8.
>> To attach a usb device through pvusb way, it should be pointed to
>> connect to which controller and which port.
> 
> I guess what I'm missing is how do I create this controller? I saw
> nothing in the guest cfg which would allow me to create one.
> 
> Is there some way to say "I don't care, find a controller and use it"?

This isn't documented, but if you set "ctrl" to -1, the code as written
will automatically:
 * find an empty port on a controller, if there is one
 * create a controller if there isn't one.

I meant to mention this in my mail yesterday though -- I think probably
there should be a defined constant in the IDL (LIBXL_USBCTRL_AUTO or
something) you should use for that, rather than just remembering a magic
value.

>>>> + 
>>>> @@ -547,6 +578,7 @@ libxl_domain_config = Struct("domain_config", [ 
>>>>      ("disks", Array(libxl_device_disk, "num_disks")), 
>>>>      ("nics", Array(libxl_device_nic, "num_nics")), 
>>>>      ("pcidevs", Array(libxl_device_pci, "num_pcidevs")), 
>>>> +    ("usbs", Array(libxl_device_usb, "num_usbs")), 
>>>  
>>> So, I'm unsure how this interacts with the controllers, which it doesn't 
>>> seem to be possible to specify at domain build time.
>>
>> In domain config, user only needs to specify usb=['2-1.6'], by default, it 
>> will
>> create a default usb contoller, and probe the 1st available controller:port 
>> for
>> the usb device to attach. So, it can work to specify usbs here only.
>>
>> Reason didn't include controller in libxl_domain_config: for HVM qemu 
>> emulated
>> usb device, all work is done in qemu (create usb controller and attach usb 
>> device),
>> no controller exists in libxl in that case.
> 
> OK, so it's an HVM only thing. I think that makes sense, but then how
> does the libxl_device_usb.ctrl field make sense or how do I use it?

Well for one, you can use libxl_device_usbctrl_add() to make a new one
on a running VM; then you can use libxl_device_usb_add() to attach it.
(These are exposed in xl as usb-ctrl-attach and usb-attach.)

 -George

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