Re: [Xen-devel] PV drivers on HVM using Xen 4.1.1

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 27 Oct 2011, Alex Bligh wrote:
> I think I've found one reason why we can't get PV block drivers on HVM
> domUs working on Xen 4.1.1 - whether we use a pvops or a xenlinux
> domU kernel.
> 
> We are using a line like:
> 
>   disk = [ "tap:aio:/tmp/centos-pvd.img,xvda,w" ]

If this is an HVM guest, you should use hda here because you want to
make sure that an emulated IDE disk is created for you as well.
Also if /tmp/centos-pvd.img is a raw file, you might as well use file:
rather than tap:aio.


> We are using effectively a standard 3.0 kernel. Config options including
> the word XEN are below.
> 
> We do not see blktapctrl starting in Xen 4.1.1 dom0, but it does
> start in Xen3. In fact as far as I can tell the Ubuntu Xen4 package
> does not contain blktapctrl at all (which would explain why it doesn't
> start). Do we need this?
> 
> It has been suggested that we don't need this, but we do need a kernel
> module that provides blktap.
> 
>   http://www.vr.org/knowledgebase/1112/Xen-Paravirt-Ops.html
> 
> suggests these might be queued for the non-existent "2.6.40", but
> it's suggested these aren't in 3.0.
> 
>   http://wiki.xensource.com/xenwiki/XAPI_on_debian
> 
> suggests there are dmks modules available, but that blktap is currenly
> only 32 bit. Can that be correct?
> 
> Isn't this what blkback does?

blkback only works with block devices. You can still use blkback if you
configure a loop device:

losetup /dev/loop0 /tmp/centos-pvd.img

and then:

disk = [ "phy:/dev/loop0,hda,w" ]

However is not going to be very fast unfortunately.

As an alternative you could download and configure upstream qemu with
linux aio and then use it as device model or just block backend but
unfortunately it needs some hacking on 4.1 (on 4.2 is semi automated
now).

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