Re: [Xen-devel] Re: [PATCH] qemu-xen: Fix PV segfault

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

On Wed, Jul 02, 2008 at 03:50:15PM +0100, Ian Jackson wrote:
> John Levon writes ("Re: [Xen-devel] Re: [PATCH] qemu-xen: Fix PV segfault"):
> > Oh, if it's just the PV bits, then it's probably part of the work to
> > drop the need for the Xen daemons altogether and move to a domain ==
> > running qemu implementation as Dan Berrange presented at the summit some
> > time ago.
> 
> I wasn't around for that summit presentation but for the record my
> personal view is that this is a bad idea.  dom0 host processes are
> much more fragile (much more vulnerable to failures induced both
> inside that process and from the rest of dom0) than a Xen domain.
> Ideally the proper functioning of guests would not depend on that kind
> of complexity.
> 
> Indeed currently even if dom0 entirely stops running user code for
> some reason, it is still possible to have PV guests keep running and
> cleanly shut themselves down (although management functions like
> migration, device hotplug, and requesting shutdown from dom0 are of
> course unavailable).
> 
> So guests should continue to be regarded as owned and parented by the
> hypervisor, not by some dom0 userland process.

My work didn't change who owned the VM's at all. I literally just took
the the domain setup code from the constructDomain() method in XenDomainInfo.py
and replicated it in QEMU. So instead of having XenD construct the domain,
QEMU did it directly. The split of responsibilities between HV vs Dom0
remained the same - it was just shuffling responsibilities within Dom0.

The original patches (no longer working against current xen-unstable)
are here

   http://hg.berrange.com/tools/xenite--devel

The main problem I didn't solve was correct cleanup in the event that
QEMU crashed - something XenD would ordinarily do - but would have to
be done by whatever was launching QEMU, or perhaps via a Dom0 kernel
hook.

Regards,
Daniel
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