[libvirt] [PATCH] Support VCPU hotplug in QEMU guests

[Daniel Veillard]

On Tue, Feb 16, 2010 at 12:12:31PM +0000, Daniel P. Berrange wrote:
> QEMU has a monitor command 'set_cpu' which allows a specific
> CPU to be toggled between online& offline state. libvirt CPU
> hotplug does not work in terms of individual indexes CPUs.
> Thus to support this, we iteratively toggle the online state
> when the total number of vCPUs is adjusted via libvirt
> 
> NB, currently untested since QEMU segvs when running this!
> 
> * src/qemu/qemu_driver.c: Toggle online state for CPUs when
>   doing hotplug
> * src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h,
>   src/qemu/qemu_monitor_json.c, src/qemu/qemu_monitor_json.h,
>   src/qemu/qemu_monitor_text.c, src/qemu/qemu_monitor_text.h: Add
>   monitor API for toggling a CPU's online status via 'set_cpu

> +static int qemudDomainHotplugVcpus(virDomainObjPtr vm, unsigned int nvcpus)
> +{
> +    qemuDomainObjPrivatePtr priv = vm->privateData;
> +    int i, rc;
> +    int ret = -1;
> +
> +    /* We need different branches here, because we want to offline
> +     * in reverse order to onlining, so any partial fail leaves us in a
> +     * reasonably sensible state */
> +    if (nvcpus > vm->def->vcpus) {
> +        for (i = vm->def->vcpus ; i < nvcpus ; i++) {
> +            /* Online new CPU */
> +            rc = qemuMonitorSetCPU(priv->mon, i, 1);
> +            if (rc == 0)
> +                goto unsupported;
> +            if (rc < 0)
> +                goto cleanup;
> +
> +            vm->def->vcpus++;
> +        }
> +    } else {
> +        for (i = vm->def->vcpus - 1 ; i >= nvcpus ; i--) {
> +            /* Offline old CPU */
> +            rc = qemuMonitorSetCPU(priv->mon, i, 0);
> +            if (rc == 0)
> +                goto unsupported;
> +            if (rc < 0)
> +                goto cleanup;
> +
> +            vm->def->vcpus--;
> +        }
> +    }
> +
> +    ret = 0;
> +
> +cleanup:
> +    return ret;
> +
> +unsupported:
> +    qemudReportError(NULL, NULL, NULL, VIR_ERR_OPERATION_INVALID, "%s",
> +                     _("cannot change vcpu count of an active domain"));
> +    goto cleanup;
> +}

  Hum, seems QEMu would allow CPU 0 and 2 to be online and CPU 1 to be
  offline for example but our code really assumes all CPUs from 0 to
  vm->def->vcpus are always online (and other offline). I hope this
  restriction we impose won't be a problem later.

[...]
> +int qemuMonitorJSONSetCPU(qemuMonitorPtr mon,
> +                          int cpu, int online)
> +{
> +    int ret;
> +    virJSONValuePtr cmd = qemuMonitorJSONMakeCommand("balloon",
> +                                                     "U:cpu", (unsigned long long)cpu,
> +                                                     "s:state", online ? "online" : "offline",
> +                                                     NULL);
> +    virJSONValuePtr reply = NULL;
> +    if (!cmd)
> +        return -1;
> +
> +    ret = qemuMonitorJSONCommand(mon, cmd, &reply);
> +
> +    if (ret == 0) {
> +        /* XXX See if CPU soft-failed due to lack of ACPI */
> +#if 0
> +        if (qemuMonitorJSONHasError(reply, "DeviceNotActive") ||
> +            qemuMonitorJSONHasError(reply, "KVMMissingCap"))
> +            goto cleanup;
> +#endif

  Hum ... why can't we activate this now ?

Just wondering about those 2 small issues, ACK,

Daniel

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