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From: Juergen Gross <jgross@suse.com>
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Date: Wed, 3 Aug 2016 05:56:45 +0200
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Subject: Re: [Qemu-devel] [PATCH] xen: use a common function for pv and hvm
 guest backend register calls
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To: Stefano Stabellini <sstabellini@kernel.org>
Cc: qemu-devel@nongnu.org, xen-devel@lists.xensource.com, kraxel@redhat.com, anthony.perard@citrix.com

On 02/08/16 20:27, Stefano Stabellini wrote:
> On Tue, 2 Aug 2016, Juergen Gross wrote:
>> Instead of calling xen_be_register() for each supported backend type
>> for hvm and pv guests in their machine init functions use a common
>> function in order not to have to add new backends twice.
>>
>> This at once fixes the error that hvm domains couldn't use the qusb
>> backend.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> Is it on purpose the qnic and vfb backends are not registered for HVM?
> 
> Yes, it is on purpose: there is no code in any toolstacks to use qnic,
> and the presence of vfb can cause problems to Linux HVM guests (or at
> least it used to), additionally vfb for HVM guests is also disabled in
> libxl.
> 
> In general, it is a good idea to disable code that is not supposed to be
> used.
> 
> Can qusb be used with HVM guests with libxl/xl?

Yes. You have to specify "type=qusb" for usbctrl, then it will work.
I have verified that.


Juergen