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

[Alex Bligh <alex@xxxxxxxxxxx>]

Ian,

--On 27 October 2011 14:44:49 +0100 Ian Campbell <Ian.Campbell@xxxxxxxxxx> 
wrote:

> > We do not see blktapctrl starting in Xen 4.1.1 dom0, but it does
> > start in Xen3.
> Are you changing dom0 kernel as well as hypervisor when you say "3" and
> "4.1.1" or is the dom0 kernel constant?
No, the Xen3 kernel is 2.6.18 Centos, and the Xen 4 kernel is 3.0
Ubuntu Oneiric.

> >  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?
> AFAIK tap:aio: usually requires blktap (both kernel side and userspace).
> There is also a block backend in qemu which can be by xl used under some
> circumstances when blktap is not available but the same is not true of
> xend.
We are using xl (though have tried xend).

> XCP (from whence that blktap version comes) is only developed/test using
> 32 bit. Taht's not to say 64 bit doesn't work, but it might not.
OK. The idea here is a production xen4.1 system, so "might not work"
sounds like a bad idea :-)

> > Isn't this what blkback does?
> blkback can only export raw block devices to guests.
We will in production use raw block devices, so that's ok. We were
just using files for testing. Is the same restriction there is
Xen3.3, because it works there (possibly for the reason below).

> A raw image can
> also be mounted with a loop back device which itself can be exported via
> blkback. Some toolstacks (xend) can do this automatically (but won't for
> a tap:aio: AFAIK) but xl will not.
I think xl /must/ be doing this, or it is difficult to explain the
behaviour below.

We discovered Ubuntu's Xen 4.1 hypervisor package does not contain
blktapctrl, so we copied that across from a build from source, plus some
libraries, and did an mknod on /dev/xen/blktap0 (which it appeared to want
to open). pvdrivers then appeared in the guest. But we then reversed
everything we'd done (we think), and pvdrivers continued to appear. We are
reinstalling to find out exactly what it was that changed.

I must admit to remaining almost totally confused as to how this is
meant to work, but thanks for your help thus far!

--
Alex Bligh

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