Re: [Xen-devel] Debian Kernel: Xen-fb-frontend as a module?

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 24 Sep 2014, David Vrabel wrote:
> On 24/09/14 11:23, Stefano Stabellini wrote:
> > On Tue, 23 Sep 2014, Konrad Rzeszutek Wilk wrote:
> >> On Tue, Sep 23, 2014 at 04:57:26PM +0100, Ian Campbell wrote:
> >>> On Tue, 2014-09-23 at 16:43 +0100, David Vrabel wrote:
> >>>> On 23/09/14 15:30, Ian Campbell wrote:
> >>>>> create !
> >>>>> title it "30s delay loading xenfb driver on some systems"
> >>>>> owner it Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> >>>>> thanks
> >>>>>
> >>>>> Hi James,
> >>>>>
> >>>>> Some of the other Xen devs were discussing an issue which sounded
> >>>>> awfully similar to this one, so I am copying the xen-devel list and
> >>>>> creating a Xen bug to track the issue, please CC xen-devel (no need to
> >>>>> subscribe but you may be moderated the first time), since the tracker
> >>>>> slurps mails from the list.
> >>>>>
> >>>>> I'm not sure of the details of the other issue but it involved
> >>>>> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=b1a3b1c8a8d963424c4699efa64dd8986b2f76d7
> >>>>>  hopefully Konrad or one of the others will follow up.
> >>
> >> That was with an HVM guest running under Xen 4.1 in which this
> >> guest config was used:
> >>
> >> vfb = ['type=vnc,vncunused=1,vnclisten=0.0.0.0']
> >>
> >> Xend would create an XenStore keys for the PV framebuffer and also making
> >> sure that QEMU VGA driver was running. The end result was that the guest
> >> would boot up to Xorg VGA driver, but the frame buffer console (so from
> >> the moment GRUB2 started Linux up to Xorg) would try to use the 
> >> xen-fbfront.
> >>
> >> And since this is HVM guest and VNCviewer was slurping contents from the
> >> VGA buffer - which was not used at all - we wouldn't get anything.
> >>
> >> Reverting the above patch fixed the issue.
> > 
> > If you have a vfb line in your config file, aren't you actually
> > explicitly requesting a vfb frontend/backend pair?
> > XenD is just doing what the user asked him to do, I wouldn't call this a
> > bug.
> > 
> > What is strange is that given that there is no running vfb backend for
> > HVM guests in a regular Xen 4.1 installation, xen-fbfront could never
> > reach the "connected" state ("4" on xenstore): so why is Linux trying to
> > use vfb when the frontend is not even connected?
> > 
> > The reason is that xenfb_probe calls register_framebuffer and
> > xenfb_make_preferred_console too early, before even knowing if the
> > backend is alive.
> > 
> > I would move the register_framebuffer and xenfb_make_preferred_console
> > calls in xenfb_backend_changed, case XenbusStateConnected. That should
> > fix it.
> 
> I don't think that will help.  Registering the frontend driver will
> still wait for CONNECTED, regardless of whether the frame buffer will
> actually be used.

It will wait for a long time but then it should work.
The problem of the long wait time is a different matter. To solve that
couldn't we simply reduce the wait time?

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