Re: [Xen-devel] [PATCH v2 01/11] x86/domctl: Add XEN_DOMCTL_set_avail_vcpus

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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.ostrovsky@xxxxxxxxxx>
>> Acked-by: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
>> ---
>> 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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