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

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

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.


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