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

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

On 05.04.2012 13:02, 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

That would work as well. The downside being that this modification then has to
go in any kernels between 3.1 and the patch. The approach from the other side
seemed to make sense so far as it changes generated output that seems targeted
only at talking to xend. The xen-xm format looks to be usable for xl too. But I
would suspect that whenever libvirt starts to support xen api/xl/libxen it will
be using a different interface which then could make use of vfb and vkbd.

Of course that does not make the change in the kernel less valuable. It prevents
the wait time whenever someone manually configures things with vfb.
It just seems to be useless to generate a config that has no use for an
interface that does not support it.

Stefan

> 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
> come up with a patch? Maybe separating non-essential from essential
> devices to have two wait loops is not feasible?
> 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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