Re: [Qemu-devel] [qemu-devel][RFC] Enable usb with default options

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 06/07/2012 11:16 AM, Benjamin Herrenschmidt wrote:
> On Thu, 2012-06-07 at 10:52 +0200, Hans de Goede wrote:
>>> At least that would improve the situation in the long run...
>>>
>>> Side note: It might be able to add an option at least to OHCI and EHCI
>>> to relax a bit the correctness of the emulation and dramatically reduce
>>> the number of exits.
>>>
>>> One way to do that is to simply reduce the timer by a factor of 10 or
>>> 100 (and increment frame numbers in bulk). This could be reasonably
>>> safely done when SOF interrupts aren't enabled, especially when only HID
>>> devices are on the bus. We could alternatively schedule additional
>>> arbitrary frames in between whenever a HID event is actually present.
>>>
>>
>> We (me and Gerd Hoffmann) have investigated that, and experimented with it.
>> It will work for simply devices, but isochronous devices break. The real
>> solution at least for x86 vms is to get the XHCI emulation finished, as
>> the XHCI controller has a much nicer hw interface from an emulation pov,
>> and it can handle usb 1-3 devices.
>
> I absolutely agree, but it will take some time for xhci to trickle
> (especially since most powerpc distros today don't have it guest side,
> we only recently endian-fixed the driver).
>
> In the meantime, this approach you experimented with would be very
> useful for us  in the common case where there is no isochronous device.
> It shouldn't be too hard for the emulator to switch back to "normal"
> frames if an ISO EP is present, no ?

That should be possible yes. Note though that although slowing down the
timer will only break isoc. stuff, it will for example also slow down
usb mass storage devices.

> That would solve the 99% case of just keyboard and mouse on all those
> machines.
>
> Do you have some patches around from those earlier experiments that we
> could try to revive, at least for OHCI ?

No, I only did this for EHCI, which may still have an option to enable
this today. Gerd may have done this for UHCI too, Gerd ?

Regards,

Hans