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

[Stefan Bader]

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:

static virDomainObjPtr
libxlDomObjFromDomain(virDomainPtr dom)
{
    virDomainObjPtr vm;
    libxlDriverPrivatePtr driver = dom->conn->privateData;
    char uuidstr[VIR_UUID_STRING_BUFLEN];

    vm = virDomainObjListFindByUUID(driver->domains, dom->uuid);
    if (!vm) {
        virUUIDFormat(dom->uuid, uuidstr);
        virReportError(VIR_ERR_NO_DOMAIN,
                       _("no domain with matching uuid '%s' (%s)"),
                       uuidstr, dom->name);
        return NULL;
-   }
+   } else {
+       if (dom->id != vm->def->id) {
+           VIR_WARN("libxlDomObjFromDomain: domid changed (%d->%d)",
+                    dom->id, vm->def->id);
+           dom->id = vm->def->id;
+       }
+   }

    return vm;
}

Not really sure this is the right way to go. Also because that warning above
happens a lot more (repeatedly) than I would have expected. This may or may not
be related to the next question.

Not a problem but more a confusion on my side: virGetDomain() has commentary
that says it will return either a pointer to an existing object or one to a new
one. Its just, I cannot see that done in the code. Is that a future plan or was
that way but was removed?

I must admit I have not looked deeper into that but one explanation of the
repeated domid mismatch would be if the virDomainPtr that virt-manager uses to
communicate with libvirt would be cloned before doing things inside libvirt. And
holding that handle for so long and not obtaining a fresh one through the
various lookup interfaces each time could be a mistake of virt-manager.
Unfortunately one that needs to be handled in some way.

-Stefan

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