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

[Ian Campbell]

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)

The vfb stuff cannot, AFAIK, ever work without a gnttab or equivalent
and therefore without this kernel feature there is no chance of it
working. I'm afraid that what's needed is for a FreeBSD kernel hacker to
work on making this stuff work :-/

Ian.
-- 
Ian Campbell
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