Re: [Xen-devel] Correct format for HVM graphics

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Apr 05, 2012 at 12:02:45PM +0100, Stefano Stabellini wrote:
> On Wed, 4 Apr 2012, Stefan Bader wrote:
> > Hi Stefano,
> > 
> > quite a while back in time, you and Konrad had a discussion about some HVM 
> > setup
> > problems via libvirt. One part was graphics and the problem seemed to be 
> > that
> > when creating a new instance through xend for HVM, the use of vfb was 
> > wrong. It
> > mostly does work but then also defines a vkbd which takes a long time in the
> > xenbus setup to finally fail.
> > 
> > Because this was not a really fatal problem it did take a long time to 
> > actually
> > get back to it. But now I had a look and found that libvirt indeed does use 
> > the
> > vfb form for both the xen-xm and xen-sxpr formats (the latter being used to
> > create guests). The decision is made based on the xend version number in 
> > the HVM
> > case. Which would be wrong if I did understand your reply correctly.
> > 
> > I have been testing a patch to libvirt, which would not use a vfb definition
> > whenever HVM is used (regardless of xend version). And it does seem to work 
> > (xm
> > list -l however has a vfb device definition, but the same happens when 
> > creating
> > the instance with a xm style config file that definitely has no vfb section 
> > in
> > it). But I am testing based on our 12.04 release which uses Xen 4.1.2. So I 
> > want
> > to make sure the solution for libvirt is correct for even the current Xen 
> > version.
> > 
> > So in short, is this always correct?
> > 
> > if (HVM or (PVM when xend is from xen < 3.0.4 / xend version < 3))
> >    do not define a vfb device
> > else /* PVM and xend version >= 3 */
> >    define a vfb device
> 
> vkbd and vfb can be considered optimizations for PV on HVM guests.
> No PV on HVM guest that I know should be able to use vfb right now, but
> Linux should be able to use vkbd since:
> 
> commit 5f098ecd4288333d87e2293239fab1c13ec90dae
> Author: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> Date:   Mon Jul 4 19:22:00 2011 -0700
> 
>     Input: xen-kbdfront - enable driver for HVM guests
>     
>     Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>     Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>     Signed-off-by: Dmitry Torokhov <dtor@xxxxxxx>
> 
> XL in xen-unstable enables vkbd for HVM guests so that you can have
> keyboard and mouse without usb emulation (that eats significant
> resources in dom0).
> 
> That said, vkbd is just a (small) optimization, it is far more
> important to get rid of the very annoying wait time at bootup.
> Rather than messing with libvirt and xend I would fix it from the Linux
> side, see the following thread:
> 
> http://marc.info/?l=xen-devel&m=133238564132683&w=2
> 
> I think that the right thing to do would be removing the additional wait
> time for vfb and vkbd devices in the PV on HVM case. We don't want to
> completely remove vfb and vkbd support for PV on HVM guests though.
> 
> Konrad, do you agree with the last reply I sent? Do you think you can

Yes.
> come up with a patch? Maybe separating non-essential from essential

Yes. I was going to after wresting with the dom0_mem=X patches.

> devices to have two wait loops is not feasible?

It should be no trouble.

> In any case, given the current state of affairs, the first patch in the
> thread from Konrad is still better than nothing.

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