Messages in this thread | | | Date | Wed, 04 Mar 2015 15:41:07 +0100 | From | Juergen Gross <> | Subject | Re: [Xen-devel] [PATCH 3/4] usb: Introduce Xen pvUSB backend |
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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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