[PATCH] xen: prefer XS for list Domains (was Re: [libvirt] libvirt + xen 3.2.1 oddities)

[Guido Günther]

On Tue, Nov 25, 2008 at 11:39:57AM +0000, Daniel P. Berrange wrote:
> On Fri, Nov 21, 2008 at 11:13:04PM +0100, Guido G?nther wrote:
> > Hi,
> > I just ran across these oddities when using a bit more libvirt+xen:
> > 
> > 1.) virsh setmaxmem:
> > 
> > On a running domain:
> >         # virsh setmaxmem domain 256000
> > completes but virsh dumpxml as well as the config.sxp still shows the
> > old amount of memory. Looks as the set_maxmem hypercall simply gets
> > ignored. xm mem-max works as expected. Smells like a bug in the ioctl?
> 
> The setmaxmem API is not performance critical, so it sounds like we
> should first try setting it via XenD, and use Hypervisor as the
> fallback instead.
I tried that and it worked as you suggested. However, checking the "old"
method of using HV out of a sudden works too now, no idea why that
reliably failed the last time and doesn't do so now (the machine has
been rebooted in the meantime though). I keep the patched package around
in case this pops up again.

> 
> > 2.) virsh list:
> > 
> > Sometimes (didn't find a pattern yet) when shutting down a running
> > domains and restarting it I'm seeing:
> > 
> >  Id Name                 State
> > ----------------------------------
> >   0 Domain-0             running
> >   2 foo                 idle
> > libvir: Xen Daemon error : GET operation failed: xend_get: error from xen daemon:
> > libvir: Xen Daemon error : GET operation failed: xend_get: error from xen daemon:
> > libvir: Xen Daemon error : GET operation failed: xend_get: error from xen daemon:
> > libvir: Xen Daemon error : GET operation failed: xend_get: error from xen daemon:
> >  7 bar                  idle
> > 
> > Note that the number of errors the corresponds to the number of
> > shutdowns. VirXen_getdomaininfolist returns 7 in the above case.
> > virDomainLookupByID later on fails for these "additional" domains.
> 
> This is basically a XenD bug. What's happening is that the domain
> has been shutdown, and got most of the way through cleanup, as far
> as the hypervisor is concerned. But something is still hanging around
> keeping the domain from being completely terminated. In this case
> XenD takes the dubious approach of just pretending the domain does
> not exist. So libvirt sees it exists in the hypervisor, but when
> asking XenD for more data, it gets that error. This really really
> sucks.
> 
> THere's not really much we can do about it when XenD is just plain
> lieing about what exists. We explicitly don't ask XenD for the list
> of domain IDs because it is incredibly slow, hence we use the HV.
> 
> The only idea I can think of is to ask XenStore for the list of
> domain IDs. This is still dramatically faster than asking XenD,
> but not quite as fast as the Hypervisor. 
> 
> > 
> > 3.) virsh list: Duplicate domains:
> > 
> >  Id Name                 State
> > ----------------------------------
> >   0 Domain-0             running
> > libvir: Xen Daemon error : GET operation failed: xend_get: error from xen daemon:
> 2A> libvir: Xen Daemon error : GET operation failed: xend_get: error from xen daemon:
> > libvir: Xen Daemon error : GET operation failed: xend_get: error from xen daemon:
> >  14 bar              no state
> > libvir: Xen Daemon error : GET operation failed: xend_get: error from xen daemon:
> >  16 bar              idle
> > 
> > Domain 14 can't be shut down (xm list only lists domain 16).
> > 
> > Could be a similar problem as the above.
> 
> Yeha, this is almost certainly just another example of XenD not properly
> cleaning up / destroying domains. If you still have a machine which
> shows this behaviour, then I'd recommend trying this change to our Xen
> impl
> 
> In xen_unified.c, find the method xenUnifiedListDomains and make it first
> call xenStoreListDomains() and then fallback to trying HV & XenD drivers.
> If we're lucky this will help....
Yes this helps indeed, thanks a lot. Possible patch attached.
 -- Guido
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