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

From: Benjamin Herrenschmidt
Subject: Re: [Qemu-devel] [qemu-devel][RFC] Enable usb with default options
Date: Thu, 07 Jun 2012 19:16:19 +1000

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 would solve the 99% case of just keyboard and mouse on all those

Do you have some patches around from those earlier experiments that we
could try to revive, at least for OHCI ?


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