Re: [Xen-devel] Commit 3a6c9 breaks QEMU on FreeBSD/Xen

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 26 Jan 2017, Roger Pau Monné wrote:
> On Wed, Jan 25, 2017 at 11:05:29AM +0000, Roger Pau Monné wrote:
> > On Tue, Jan 24, 2017 at 01:30:02PM -0800, Stefano Stabellini wrote:
> > > On Tue, 24 Jan 2017, Stefano Stabellini wrote:
> > > > On Tue, 24 Jan 2017, Roger Pau Monné wrote:
> > > > > Hello,
> > > > > 
> > > > > The following commit:
> > > > > 
> > > > > commit 3a6c9172ac5951e6dac2b3f6cbce3cfccdec5894
> > > > > Author: Juergen Gross <jgross@xxxxxxxx>
> > > > > Date:   Tue Nov 22 07:10:58 2016 +0100
> > > > > 
> > > > > xen: create qdev for each backend device
> > > > > 
> > > > > Prevents me from running QEMU on FreeBSD/Xen, the following is 
> > > > > printed on the
> > > > > QEMU log:
> > > > > 
> > > > > char device redirected to /dev/pts/2 (label serial0)
> > > > > xen be core: xen be core: can't open gnttab device
> > > > > can't open gnttab device
> > > > > xen be core: xen be core: can't open gnttab device
> > > > > can't open gnttab device
> > > > > 
> > > > > # xl create -c ~/domain.cfg
> > > > > Parsing config from /root/domain.cfg
> > > > > libxl: error: libxl_dm.c:2201:device_model_spawn_outcome: Domain 
> > > > > 32:domain 32 device model: spawn failed (rc=-3)
> > > > > libxl: error: libxl_create.c:1506:domcreate_devmodel_started: Domain 
> > > > > 32:device model did not start: -3
> > > > > libxl: error: libxl_dm.c:2315:kill_device_model: Device Model already 
> > > > > exited
> > > > > libxl: error: libxl.c:1572:libxl__destroy_domid: Domain 
> > > > > 32:Non-existant domain
> > > > > libxl: error: libxl.c:1531:domain_destroy_callback: Domain 32:Unable 
> > > > > to destroy guest
> > > > > libxl: error: libxl.c:1458:domain_destroy_cb: Domain 32:Destruction 
> > > > > of domain failed
> > > > > # cat /var/log/xen/qemu-dm-domain.log
> > > > > char device redirected to /dev/pts/2 (label serial0)
> > > > > xen be core: xen be core: can't open gnttab device
> > > > > can't open gnttab device
> > > > > xen be core: xen be core: can't open gnttab device
> > > > > can't open gnttab device
> > > > > 
> > > > > I'm not really familiar with any of that code, but I think that using
> > > > > qdev_init_nofail is wrong, since on FreeBSD/Xen for example we don't 
> > > > > yet
> > > > > support the gnttab device, so initialization of the Xen Qdisk backend 
> > > > > can fail
> > > > > (and possibly the same applies to Linux if someone decides to compile 
> > > > > a kernel
> > > > > without the gnttab device). Yet QEMU can be used without the Qdisk 
> > > > > backend.
> > > > 
> > > > How did you manage to configure QEMU before? The configure script had
> > > > xc_gnttab_open calls in it up to Xen 4.6.
> > > 
> > > I know the answer! Because the configure script only compiles the code,
> > > doesn't try to run it. xc_gnttab_open compiled correctly but returned
> > > error when executed. Is that right?
> > 
> > Yes, I'm quite that's right. FreeBSD is using gnttab_unimp.c, which 
> > implements
> > xengnttab_open, so compilation will not fail.
> > 
> > > 
> > > 
> > > > I am happy to support a use case where the kernel doesn't have gntdev,
> > > > but it needs to be explicit: we need to detect it in the configure
> > > > script, then avoid the initialization of devices which require it.
> > > 
> > > I would still prefer configure to be able to detect this case. If it
> > > cannot be made to detect it, then we can try to figure out a way to
> > > catch the initialization errors at run time. 
> > 
> > I think it's better to simply fail to initialize Xen Qdisk at runtime, or 
> > else
> > a xen-tools/QEMU compiled on a non-Xen environment won't get gnttab and as a
> > consequence Xen Qdisk support enabled, and I think it's quite common for
> > distros to compile Xen packages on non-Xen environments (where 
> > /dev/xen/gnttab
> > is not available).
> 
> Ping?

Sorry for the delay. Obviously, you are correct.


> I'm not really sure how to solve this because I have zero experience with QEMU
> internals (all this qdev stuff). Can we restore the previous behavior, where
> the failure to initialize a device wouldn't prevent QEMU from starting?

I am convinced that we need to be able to handle the initialization
errors at run time, but I would prefer not to revert
3a6c9172ac5951e6dac2b3f6cbce3cfccdec5894.

It should be just a matter of replacing qdev_init_nofail with something
that can fail. I couldn't find a regular qdev_init that can return
error, so maybe we would need to add it.

Juergen, would you be up for coming up with a patch?

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