Re: [Xen-devel] qemu and xl semantics

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Wed, 2010-12-22 at 15:42 +0000, Christoph Egger wrote:
> On Monday 20 December 2010 11:23:59 Ian Campbell wrote:
> > On Fri, 2010-12-17 at 17:21 +0000, Christoph Egger wrote:
> > > On Friday 17 December 2010 11:32:41 Ian Campbell wrote:
> > > > On Fri, 2010-12-17 at 09:49 +0000, Christoph Egger wrote:
> > > > > On Friday 17 December 2010 10:15:14 Ian Campbell wrote:
> > > > > > On Fri, 2010-12-17 at 09:00 +0000, Christoph Egger wrote:

> > The only recent qemu change which I am aware of in this area is the
> > backport of the blkback functionality from upstream Qemu. However this
> > should only be enabled in cases where blkback or blktap are not
> > available and furthermore is tied to libxl/xl so shouldn't have done
> > anything on xend (although shouldn't != doesn't).
> 
> Is this supposed to interact with the Dom0 PV disk backend driver ?

The qemu disk backend is intended to be used when there is no dom0 PV
disk backend driver in the kernel, if there is a driver in the kernel
then it is intended that the kernel driver should be used.

The background to this is that we have just gotten the basic dom0
support into the upstream Linux kernel and are about to start on
upstreaming the backend drivers. However we observed that there may not
be any need to upstream a block backend since a userspace implementation
may well suffice. It's not a done decision but the early signs are good,
especially compared with blktap which hits userspace anyway.

Even if we do eventually decide to upstream the Linux blkback using the
disk backend in qemu tides us over for the time being.

> > > xbdback backend/vbd/1/832: can't VOP_OPEN device 0xe13: 16
> > > 16 means EBUSY.
> >
> > But it works other than this message?
> 
> Well, that means that there are now two processes which want to open the vbd
> simultaneously. The first one wins. And on guest shutdown the VOP_CLOSE fails.

Right. This suggests that the qemu disk backend is being erroneously
activated on NetBSD when what you actually want is to use the xbdback
process. I think you likely need to patch libxl to do the right thing
for NetBSD, currently the decision is based on the presence or absence
of blktap, I guess it needs extending to detect NetBSD's backend too.

Alternatively perhaps NetBSD also wants to transition to using the qemu
based block backend, I suppose there is benefit to be had from sharing
this code between Linux and NetBSD dom0.

> I did some more debugging and figured out xenbackendd runs the vif-bridge
> script so network with a PV driver works. 
> 
> The scripts not running is the one associated with 'vbd' and qemu-ifup.
> AFAIK  qemu-dm always run qemu-ifup which is no longer the case.
> 
> qemu-ifup adds tap interface to the bridge.

This changed (on Linux) quite a while ago so I didn't think of it.
qemu-ifup is not used there any more and instead the vif-bridge script
is called for both PV and TAP devices. See xen-unstable.hg
21944:0232bc7c9544, I guess either some equivalent change is needed to
tools/hotplug/NetBSD or the libxl bit needs to be conditional on NetBSD.

> local = ""
>  domain = ""
>   0 = ""
>    backend = ""
>     qdisk = ""

You probably don't want this on NetBSD as described above.

However AFAIK only libxl will cause these to be created, so if you are
using xend then I am confused.

Ian.


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel