Re: [Xen-devel] USB passthrough burning 5% of CPU?

[George Dunlap <dunlapg@xxxxxxxxx>]

On Mon, Aug 5, 2013 at 3:01 PM, Gordan Bobic <gordan@xxxxxxxxxx> wrote:
> On Mon, 5 Aug 2013 14:57:09 +0100, George Dunlap <dunlapg@xxxxxxxxx> wrote:
>>
>> On Mon, Aug 5, 2013 at 1:28 PM, Gordan Bobic <gordan@xxxxxxxxxx> 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.

You could probably verify this by taking a xentrace and then running
xenalyze on it.  xenalyze can be found here:

http://xenbits.xensource.com/ext/xenalyze.hg

There's a very minimalist .html doc in the repo.  You want "--summary
--with-mmio-enumeration" I think.

It may be that the device you're passing through is more "active" wrt polling...

 -George

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