[Pkg-xen-devel] Bug#658100: Bug#658100: xl: hangs in qemu-dm when trying to run PV domU

[Witold Baryluk]

On 01-31 19:58, Ian Campbell wrote:
> On Tue, 2012-01-31 at 18:41 +0100, Witold Baryluk wrote:
> > On 01-31 16:51, Ian Campbell wrote:
> > > On Tue, 2012-01-31 at 12:32 +0100, Witold Baryluk wrote:
> > > > I found NetBSD user/developer had very similar problem (when using xl as TOOLSTACK,
> > > > just like me!) about year ago (Mar 2011).
> > > > 
> > > > http://mail-index.netbsd.org/port-xen/2011/03/31/msg006552.html
> > > > with some discussion, that NetBSD doesn't have gnttap implemented in kernel:
> > > 
> > > I think that is correct and I'm afraid this will prevent the use of
> > > either vfb or qdisk (which is what file:// becomes in the absence of
> > > blktap, which freebsd does not have) disk backends. Userspace PV device
> > > backends simply cannot work without a gnttab device.
> > > 
> > > There has been some work on xen-unstable (by Roger Pau Monet) to make
> > > file:// type disks use the in kernel vbd backend (which won't need vbd)
> > > but this is still a work in progress and won't be in 4.1. Hopefully this
> > > will be in in time for 4.2.
> > > 
> > > As a workaround in the meantime you can use the FreeBSD equivalent of
> > > losetup (vnconfig?) to manually turn your file:/path/etc into a
> > > phy:/dev/vndN which will use the in kernel vbd backend.
> > > 
> > > (I'm not a FreeBSD guy so I may have got some of the names above wrong)
> > 
> > 
> > I do not know what you are talking about. Im using Debian's Xen and
> > Debian/GNU Linux 3.2 kernel as dom0. The same happens with freebsd and linux
> > kernels as domU. Clearly I expect Linux dom0 (and domU) to support
> > gnttab. (btw. there is NO dom0 support in FreeBSD).
> 
> Your bug report said:
> movax-dev:/etc/xen# dpkg -l | grep -- -image
> ii  gnumach-image-1-xen-486              2:1.3.99.dfsg.git20111010-1  The GNU version of the Mach microkernel
> ii  gnumach-image-1.3.99-xen-486         2:1.3.99.dfsg.git20111010-1  The GNU version of the Mach microkernel for Xen
> ii  kfreebsd-image-9-xen                 9.0-1                        kernel of FreeBSD 9 image (meta-package)
> ii  kfreebsd-image-9.0-1-xen             9.0-1                        kernel of FreeBSD 9.0 image
> [...]
> 
> so I'm sure you can see where I got the idea you might be running
> Debian/kfreebsd from.
> 

Indeed, I may not stress this sufficiently in initial post. (there was uname -a
and some host information, at the end, but I probably should start from it)

> [...]
> > I think this is because /dev/xen/gntdev file was provided by xen-evtchn
> > or xenfs module in previous kernels (like 2.6.32 in squeeze), but is now
> > in separate module (xen-gntdev).
> > 
> > 
> > I performed 'lsmod' after reboot, and there was NO single xen releated
> > module loaded, even after 'xl create ...'. So clearly something is not doing
> > its job. I guess, 'xl' tool should make sure proper modules are loaded,
> > or /etc/init.d/xen(d) should modprobe them.
> 
> $ grep modprobe /etc/init.d/xen*
> /etc/init.d/xend:	modprobe xenfs 2>/dev/null
> /etc/init.d/xend:	modprobe xen-evtchn 2>/dev/null
> 
> so it is loading some things but clearly it needs to load some more,
> like xen-gntdev and xen-*back.
> 

I guess, even with xl toolstack, a some parts of /etc/init.d/xend will
be started, like modprobing, but is not yet done in sid.

I think it is good to add this modprobes, but how about checking also in
qemu-dm and quiting with error when it cannot open this devices in
/dev/xen/ ? Currently qemu-dm is busy-looping there infinitly, using
100% CPU, and without giving good error message. Why qemu-dm is
busy-looping anyway? Should there be some exit(1) or sleep(1) ?


> > I loaded xen-gntdev xen-evtchn xen-blkback xen-netback (not in this
> > order) manually, and now everything works.
> 
> Great.

Hope to see this changes in xen-utils-common soon.

Good work! I'm using Debian/Linux Xen on few machines for like 5 years now,
and it is awesome. Thanks. :)


Regards,
Witek

-- 
Witold Baryluk
JID: witold.baryluk // jabster.pl