[Qemu-devel] Re: UHCI idle detection

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Jan 4, 2011 at 12:13 PM, Gerd Hoffmann <kraxel@redhat.com> wrote:
> On 01/04/11 12:48, Stefan Hajnoczi wrote:
>>
>> CPU utilization is a known issue with UHCI emulation.  I spent a short
>> time poking around the code and USB specifications trying to come up
>> with a way to detect "idle" periods where we don't need to poll the
>> frame list at 1000 Hz.
>
> Check out
>
> http://cgit.freedesktop.org/spice/qemu/log/?h=usb.3
>
> It adds remote wakeup support to uhci, hub and hid emulation.  This allows
> the guest to suspend the usb bus when it is idle, the 1000 Hz wakeup goes
> away then.
>
> Only problem with that is guests don't do that by default because there is
> way too much broken hardware out there.  That includes the current qemu usb
> hid emulation which advertises remote wakeup support although it isn't
> implemented :(
>
> Windows guests needs some registry hackery and Linux guests some udev rules
> to enable remote wakeup permanently.

That commit inspired me to look at UHCI.  If the solution requires
modifying the guest then it is not widely useful.

>> When the guest writes to those memory pages again in order to issue a
>> new USB transaction, we catch the write.  UHCI unprotects the guest
>> memory pages and turns the frame timer back on.
>
> It isn't that simple.  "idle" state depends on the usb devices too. With a
> usb tablet connected (and no remote-wakeup being used) uhci will poll the
> tablet in regular intervals and the usb tablet will either send the next
> event in the queue or NACK the transfer in case there is no event available.

Is there something preventing USB tablet emulation from using a
non-polling approach?  Devices need to be able to kick UHCI when data
becomes available.

Stefan