Re: [Xen-devel] PATCH: Enable QEMU booting of blktap disks

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Thu, Jul 19, 2007 at 10:34:12AM -0700, Andrew Warfield wrote:
> So two comments on this:
> 
> In the other thread that's currently going on this topic, it sounds
> like others are quite successfully using the phantom code.  Why is it
> broken for you?

I really can't see how it works for anybody in 3.1.0 since the code which
sets up phantom devices simply doesn't work

        try:
            imagetype = self.vm.info['image']['type']
        except:
            imagetype = ""

        if imagetype == 'hvm':

The body of that try: statement is trying to read hash keys which don't
exist, since 'vm.info' isn't a hash. So imagetype is always "" and so
none of the phantom setup code ever gets run.  Even once fixing that I
never get any devices appearing and the Vm just immediately shuts down.
It seems to be looking for the /dev/xvd* device nodes in Dom0 rather
than DomU which seems rather wrong.

> As I've said before, I dislike the idea of having separate
> implementations of disks -- one in qemu and one in tapdisk.  We'd
> quite like to encourage people to be able to extend virtual block
> devices in the future, and it seems like your approach is going to
> force them to do two independent implementations of things.  It also
> leads to complications if you want to add things like caching, shared
> ramdisks, etc.  If phantom is broken, why don't we just fix that?

AFAICT with or without  my change you need to have two separate impls
of every disk format, since the phantom device stuff is only ever used
by blktap - non blktap disks still get processed directly by QEMU. Now
if we intend to remove all support for file: entirely, and make blktap
compulsory for file backed VMs then I can see the benefit in having 
everything go via one codepath. Though now having 2 userspace daemons
in Dom0 per HVM guest seems like its going in wrong direction to me.

IMHO the entire design & impl of blktap userspace was broken from the
start because it is duplicating functionality already in the QEMU 
codebase. With the benefit of hindsight, I would suggest that it would
be better to have QEMU able to speak the native blktap protocol straight
to the blktap kernel driver. Keep HVM using QEMU for all file backed 
disks, since it already handles all the formats just fine, and have a
new machine type in QEMU for paravirt VMs which provided the tap daemon
replacement and also a PVFB daemon replacement. The you could kill the
entire blktap userspace codebase & most of the PVFB userspace codebase
and the libvncserver requirement. 

So there'd only be 1 single daemon in Dom0 per VM, it would be the same
daemon for PV and HVM, and all the open source virt platforms (Xen, KVM, 
QEMU, VirtualBox) would all be reaping the benefit of each other's code
improvements to QEMU driver model, in particular for disk format code & 
VNC server code, rather than forking & reimplementing private copies.

Of course this isn't a quick job, but if the motiviation is reducing
code duplication & alternative I/O paths, the focusing on QEMU for 
everything seems like a much more viable idea than more Xen specific
code.

Dan.
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