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

From: Ian Campbell <ian.campbell@citrix.com>
To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
	Paul Durrant <Paul.Durrant@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>,
	"xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
	Stefano Stabellini <Stefano.Stabellini@citrix.com>,
	Ian Jackson <Ian.Jackson@citrix.com>
Subject: Re: [PATCH v5 1/6] libxl: do not add a vkb backend to hvm guests
Date: Fri, 24 Jul 2015 12:10:24 +0100	[thread overview]
Message-ID: <1437736224.24746.71.camel@citrix.com> (raw)
In-Reply-To: <alpine.DEB.2.02.1507241132120.28668@kaball.uk.xensource.com>

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@eu.citrix.com
> > > ]
> > > Sent: 24 July 2015 11:21
> > > To: Paul Durrant
> > > Cc: Stefano Stabellini; xen-devel@lists.xensource.com; 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@lists.xen.org [mailto:xen-devel-
> > > > > bounces@lists.xen.org] On Behalf Of Stefano Stabellini
> > > > > Sent: 23 July 2015 18:28
> > > > > To: xen-devel@lists.xensource.com
> > > > > 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...

Ian.