Re: USB passthrough burning 5% of CPU?

[Gordan Bobic <gordan@bobich.net>]

 On Mon, 5 Aug 2013 14:57:09 +0100, George Dunlap <dunlapg@umich.edu> 
 wrote:
> On Mon, Aug 5, 2013 at 1:28 PM, Gordan Bobic <gordan@bobich.net> 
> wrote:
>> Ever since I switched from passing a PCI USB controller
>> through to a domU in favour of passing USB devices using
>> the "usbdevice" config parameter, the domU affected
>> seems to have qemu-dm "idling" at about 5% CPU usage in
>> dom0. With the PCI USB controller passed through, this
>> does not happen.
>>
>> Is this normal/expected? Does a similar continuous CPU
>> overhead invisibly apply to PCI passthrough devices as
>> well? Or is this CPU tax only applicable to USB
>> passthrough?
>
> Do you have the emulated USB controller enabled when you're passing
> through the PCI USB controller?

 Yes, but nothing is hanging off it (well, other than maybe the
 emulated VNC mouse/keyboard).

> My understanding was that certain USB devices must, by specification,
> be polled something like a hundred times per second.  QEMU has to
> emulate every one of these.  One of the things XenServer does for
> really high-density VDI deployments, I believe, is disable USB in the
> guests entirely.

 So it stands to reason that with no physical USB devices passed through
 the USB controller polling gets optimized out.

 Gordan