[libvirt] libxl: Issues with virt-manager when used to manager Xen domains

[Daniel P. Berrange]

On Tue, Mar 25, 2014 at 04:22:54PM +0100, Stefan Bader wrote:
> This started off with some regression testing after going forward to Xen-4.4. We
> currently would pair that with a libvirt version 1.2.2 and right now operations
> through virsh seem to be working (mostly) well. But when using virt-manager (not
> the most up-to-date versions but some combinations that quite likely will occur
> (0.9.1 and 0.9.5)) there are some issues.
> 
> Two symptoms seem to be caused by the same underlying problem which is caused by
> the way virt-manager interacts with libvirt. That seems to be that virt-manager
> acquires a reference to virDomainPtr only once and then uses that for subsequent
> call. This is a problem because both virDomainPtr object and virDomainObjPtr
> objects contains a domid but only the latter gets updated.
> 
> So both, creating a new guest (which must be a define step and then use the
> handle to start the guest) as well as rebooting a guest cause problems for
> virt-manager. Mainly because of libxlDomainGetInfo() which retrieves the
> virDomainObjPtr for a given virDommainPtr. Then checks the domain object to be
> active or not but uses the domid from the domain to call into libxl:
> 
>     if (!(vm = libxlDomObjFromDomain(dom)))
>         goto cleanup;
>     ...
>     if (!virDomainObjIsActive(vm)) {
>         info->cpuTime = 0;
>         info->memory = vm->def->mem.cur_balloon;
>         info->maxMem = vm->def->mem.max_balloon;
>     } else {
>         if (libxl_domain_info(priv->ctx, &d_info, dom->id) != 0) {
> 
> This fails either with dom->id being -1 (still) or using the old id from before
> the reboot. A dirty hack seems to avoid this:

Yep, this is a pretty commonly hit problem. The solution is to *never*
access dom->id in the driver code at all. eg In this code snippet above
you could use  vm->def->id instead which should be up to date.

Regards,
Daniel
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