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

[Gordan Bobic <gordan@xxxxxxxxxx>]

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.

Gordan

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