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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 18:40:38 +1000

On Thu, 2012-06-07 at 10:32 +0200, Hans de Goede wrote:
> Hi,
> Sorry for jumping in the middle of the thread, I missed the beginning
> of this. Enabling USB by default, esp. emulating a USB mouse by default
> is a *bad* idea. They way the periodic schedule of the various USB
> controllers works means that emulating a USB device means a 1000 vm exits
> per second extra, even when the vm is completely idle! USB powermanagement
> can be used to negate this, but this is off in both Linux and Windows for
> HID devices by default because there are too many broken HID devices.

On the other hand it is the only choice of input device on some
platforms such as pseries (and MacG5 when I get to make it work :-)

Would it be possible to "whitelist" the QEMU emulated HID devices
in the kernel to enable dynamic PM for them (at least when they sit
alone on the bus) ? Or is our emulation busted too ?

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.


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