Re: [Xen-devel] [PATCH v5 1/6] libxl: do not add a vkb backend to hvm guests

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 24 Jul 2015, Ian Campbell wrote:
> On Fri, 2015-07-24 at 11:56 +0100, Stefano Stabellini wrote:
> > On Fri, 24 Jul 2015, Paul Durrant wrote:
> > > > -----Original Message-----
> > > > From: Stefano Stabellini [mailto:stefano.stabellini@xxxxxxxxxxxxx
> > > > ]
> > > > Sent: 24 July 2015 11:21
> > > > To: Paul Durrant
> > > > Cc: Stefano Stabellini; xen-devel@xxxxxxxxxxxxxxxxxxx; Wei Liu; 
> > > > Ian Jackson;
> > > > Ian Campbell
> > > > Subject: RE: [Xen-devel] [PATCH v5 1/6] libxl: do not add a vkb 
> > > > backend to
> > > > hvm guests
> > > > 
> > > > On Fri, 24 Jul 2015, Paul Durrant wrote:
> > > > > > -----Original Message-----
> > > > > > From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> > > > > > bounces@xxxxxxxxxxxxx] On Behalf Of Stefano Stabellini
> > > > > > Sent: 23 July 2015 18:28
> > > > > > To: xen-devel@xxxxxxxxxxxxxxxxxxx
> > > > > > Cc: Wei Liu; Ian Jackson; Ian Campbell; Stefano Stabellini
> > > > > > Subject: [Xen-devel] [PATCH v5 1/6] libxl: do not add a vkb 
> > > > > > backend to
> > > > hvm
> > > > > > guests
> > > > > > 
> > > > > > When QEMU restricts its xenstore connection, it cannot 
> > > > > > provide PV
> > > > > > backends. A separate QEMU instance is required to provide PV 
> > > > > > backends
> > > > in
> > > > > > userspace, such as qdisk. With two separate instances, it is 
> > > > > > not
> > > > > > possible to take advantage of vkb for mouse and keyboard, as 
> > > > > > the QEMU
> > > > > > that emulates the graphic card (the device model), would be 
> > > > > > separate
> > > > > > from the QEMU running the vkb backend (PV QEMU).
> > > > > > 
> > > > > > Removing this functionality is acceptable, because is only 
> > > > > > useful for
> > > > > > power saving when usb emulation is off, letting QEMU sleep 
> > > > > > for longer
> > > > > > periods of time.  However usb emulation is on by default, and 
> > > > > > how to
> > > > > > take advantage of this configuration has never been 
> > > > > > documented.
> > > > > > 
> > > > > 
> > > > > I don't think I agree. Turning off USB emulation for HVM guests 
> > > > > (particularly
> > > > Windows) has been shown to be highly advantageous in performance 
> > > > and
> > > > scalability terms, and we have a prototype HID driver (not yet 
> > > > part of the
> > > > XenProject driver set, but hopefully soon will be) which uses 
> > > > vkb.
> > > > 
> > > > I would appreciate if this kind of comments were made at v1 or 
> > > > v2, not
> > > > v5 of a series :-)
> > > > 
> > > 
> > > Yes, I realise that, but I've been busy... sorry.
> > > 
> > > > 
> > > > I know that turning USB emulation off is a big win, but nobody is 
> > > > really
> > > > doing it. The reason is that we didn't properly documented how to 
> > > > do it.
> > > 
> > > It's documented for XenServer and we have toolstack support to do 
> > > it.
> > 
> > You could still use it if you call libxl_device_vkb_add explicitely and
> > you avoid creating any of depriv QEMU users (xen-qemudepriv-domid* and
> > xen-qemudepriv-shared).
> 
> There really ought to be a way at the libxl level (but not necessarily
> in xl) to disable the depriv without having to arrange things "just so"
> such that it doesn't occur...

I have already introduced a device_model_user option, would that be
enough? We could check on device_model_user == root.

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