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

[Stefan Bader]

On 25.03.2014 16:36, Daniel P. Berrange wrote:
> 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.

OK, it just seems for something that should not be done, it is done plenty of
times in the libxl_driver codebase right now. At least pause and unpause seem to
do the same...

-Stefan
> 
> Regards,
> Daniel
> 


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