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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Oct 27, 2011 at 01:56:41PM +0100, 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" ]
> 
> 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.

Right, they are a no-go. There is a 
> 
>  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?

There is a 64-bit (and 32-bit) version on Daniel's git tree:

git://xenbits.xensource.com/people/dstodden/linux.git

.. but the deal is that it is unmaintained (Daniel left Citrix).

Thought interestingly .. he made a version of it where all of the fiddling
with the generic code has been removed. Neat.

> 
> Isn't this what blkback does?

Blkback can't handle files - it can only handle block devices.
> 
> domU config is below. Note the very same VM with the same config file
> works just fine on xen 3.3.1. This domU is Centos 2.6.18 with
> unmodified_drivers xenlinux type kernel (as supplied by Centos). Every
> other kernel we've tried does the same, save that modern ones also
> unplug the emulated devices so no disks appear as well.
> 
> PV nics work fine.

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