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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Aug 05, 2013 at 01:28:39PM +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 think that is about right. USB1 sucks at virtualization so
QEMU has to scan a list of memory regions to figure out if
the guest has decided that "hey, I've a got a packet of
data'. The CPU overhead for PCI passthrough does not happen
as the PCI device can detect where the request went and
do it stuff.

USB2.0 is not any better either. The XHCI on the other hand
is - but I am not sure if that is implemented in QEMU yet.


Ah, Gerd here has a nice update:
http://www.linux-kvm.org/wiki/images/b/be/2012-forum-qemu-usb-status-update.pdf

> 
> Gordan
> 
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