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

On Mon, 2013-08-05 at 13:28 +0100, Gordan Bobic 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?

I've heard that USB host controllers are expensive to virtualise because
USB has an effective 100ms (?) poll at the hardware level, which needs
to be managed (something to do with Isochronous URBs and/or the USB
schedule? I'm not 100% sure -- deeper USB voodoo than I can manage

It had never occurred to me that this might also apply to usbdevice
style passthrough but since in that case, AIUI, you are running an
emulated host controller with a real device behind it it seems plausible
that this configuration would have that sort of overhead too.


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