Re: [Xen-devel] [PATCH 3/4] usb: Introduce Xen pvUSB backend

[Juergen Gross]

On 03/04/2015 03:29 PM, Ian Campbell wrote:
> On Wed, 2015-03-04 at 14:19 +0000, David Vrabel wrote:
> > On 04/03/15 14:09, Juergen Gross wrote:
> > > The main question whether it is worth to consider this alternative is
> > > the performance aspect. Does anyone have an idea which USB devices would
> > > typically be used via pvusb? I'd suspect memory sticks and USB disks
> > > and perhaps webcams being the most performance relevant ones. Is an
> > > additional copy operation of user data acceptable here?
> >
> > I have no idea.  We (XenServer) have no use cases at all for USB device
> > passthrough.
>
> My gut feeling is that for USB 1 and 2 the bus itself isn't fast enough
> that anyone would care. qdisk has acceptable for disks, so it's probably
> ok for usb too.

While I can accept the bus speed reasoning, I doubt qdisk is copying
data between user and kernel space under normal circumstances. I think
disk I/Os are done using DMA to/from the user buffer directly.

> For usb 3 onwards, well, maybe when we care about those we'll decide
> that a kernel space driver is needed, but for now it seems like
> userspace would be ok.

Do you have another feeling about the probability of a need to do usb 3?
If it is already on the horizon I wouldn't want to do the user space
backend now and the kernel one next year. :-)


Juergen
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