On 11/09/2016 10:04 AM, Andrew Cooper wrote:
> On 09/11/16 14:39, Boris Ostrovsky wrote:
>> This domctl is called when a VCPU is hot-(un)plugged to a guest (via
>> 'xl vcpu-set'). While this currently is only intended to be needed by
>> PVH guests we will call this domctl for all (x86) guests for consistency.
>>
>> Signed-off-by: Boris Ostrovsky <boris.ostrov...@oracle.com>
>> Acked-by: Daniel De Graaf <dgde...@tycho.nsa.gov>
>> ---
>> Changes in v2:
>> * Added comment in arch_do_domctl() stating that the domctl is only required
>> for PVH guests
> I am not happy with this change until we understand why it is needed.
>
> Are we genuinely saying that there is no current enforcement in the
> PV-hotplug mechanism?
That's right. Don't call setup_cpu_watcher() in Linux and you will be
running with maxvcpus.
>
>> diff --git a/xen/arch/x86/domctl.c b/xen/arch/x86/domctl.c
>> index 2a2fe04..b736d4c 100644
>> --- a/xen/arch/x86/domctl.c
>> +++ b/xen/arch/x86/domctl.c
>> @@ -1430,6 +1430,23 @@ long arch_do_domctl(
>> }
>> break;
>>
>> + case XEN_DOMCTL_set_avail_vcpus:
>> + {
>> + unsigned int num = domctl->u.avail_vcpus.num;
>> +
>> + /*
>> + * This is currently only needed by PVH guests (but
>> + * any guest is free to make this call).
>> + */
>> + ret = -EINVAL;
>> + if ( num > d->max_vcpus )
>> + break;
>> +
>> + d->arch.avail_vcpus = num;
>> + ret = 0;
>> + break;
>> + }
> What do you actually mean by "avail_vcpus"? What happens if a vcpu
> higher than the new number is currently online and running? What
> happens to the already-existing vcpus-at-startup value?
It shouldn't happen: we set avail_vcpus at domain creation time to
vcpus-at-startup.
The name is not great. It would have been better to have it online_vcpus
but that usually means that VPF_down is set (which can happen, for
example, when the guest offlines a VCPU).
-boris
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