KVM and Xen guest fail with cirrus emulated graphic card

Daniel Stone daniel at fooishbar.org
Thu Nov 24 12:16:54 UTC 2016

Hi Dario,

On 21 November 2016 at 16:55, Dario Faggioli <dario.faggioli at citrix.com> wrote:
> So, as I reported in my comment to this (already existing) bug:
> https://bugzilla.redhat.com/show_bug.cgi?id=1227770
>  - on Wayland, with Cirrus as emulated graphic card, I don't see the
>    GNOME login screen appearing.
>  - if I keep Cirrus and disable Wayland (via /etc/gdm/custom.conf),
>    both Xen and KVM guests works ok.
>  - if I keep wayland and use a different graphic card (vga or qxl, for
>    instance), both Xen and KVM guests works ok.
> There's not much information online, and what's there suggests that it
> may be due to the hw characteristics of Cirrus (e.g., the 24bpp thing),
> which are not supported by Wayland.

As Pekka says, it's important to differentiate between Wayland (an
abstract protocol), and what I assume you're talking about which is
Mutter's implementation of a Wayland server.

> The issue has been brought up and is being discussed in this thread
> too:
> https://lists.xen.org/archives/html/xen-devel/2016-11/msg01579.html
> Now, what we're interested to know is, if possible:
>  - how we can actually figure out what the issue is
>    (I've poked around, but haven't found much in any log)?
>  - is it something that can be made to work? If yes, is
>    it likely to happen, or will this be a 'won't fix'?
> This is relevant because, as I said, for both for KVM and Xen, what
> we're getting _by_default_ is a non working guest, if Wayland is used.
> :-(

Generally speaking, I have to say I agree with ajax, even if I
would've perhaps worded it a bit more gently. ;) Packed 24bpp is going
to be pain, not least because I don't know of any clients which render
in packed-24, so you're always going to take a format conversion from
the client's 32bpp to the final 24bpp, rather than being able to do
memcpy. 1024x768 is also not ideal for 2016: good luck with a web
browser ...

The 24bpp paths in pretty much everything are also badly untested, so
that's asking for trouble. I really think you'd be far far better off
trying to figure out how to move off the legacy Cirrus emulation as
soon as you can.


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