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

From: Juergen Gross
Date: Wed Mar 04 2015 - 09:41:14 EST


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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