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From: Laine Stump <laine@redhat.com>
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Date: Tue, 6 Sep 2016 09:58:03 -0400
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Subject: Re: [Qemu-devel] [PATCH RFC] docs: add PCIe devices placement
 guidelines
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To: Gerd Hoffmann <kraxel@redhat.com>, Laszlo Ersek <lersek@redhat.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>, qemu-devel@nongnu.org, mst@redhat.com, Peter Maydell <peter.maydell@linaro.org>, Drew Jones <drjones@redhat.com>, Andrea Bolognani <abologna@redhat.com>, Alex Williamson <alex.williamson@redhat.com>

On 09/06/2016 07:35 AM, Gerd Hoffmann wrote:
> While talking about integrated devices:  There is docs/q35-chipset.cfg,
> which documents how to mimic q35 with integrated devices as close and
> complete as possible.
>
> Usage:
>   qemu-system-x86_64 -M q35 -readconfig docs/q35-chipset.cfg $args
>
> Side note for usb: In practice you don't want to use the tons of
> uhci/ehci controllers present in the original q35 but plug xhci into one
> of the pcie root ports instead (unless your guest doesn't support xhci).

I've wondered about that recently. For i440fx machinetypes if you don't 
specify a USB controller in libvirt's domain config, you will 
automatically get the PIIX3 USB controller added. In order to maintain 
consistency on the topic of "auto-adding USB when not specified", if the 
machinetype is Q35 we will autoadd a set of USB2 (uhci/ehci) controllers 
(I think I added that based on your comments at the time :-). But 
recently I've mostly been hearing that people should use xhci instead. 
So should libvirt add a single xhci (rather than the uhci/ehci set) at 
the same port when no USB is specified?