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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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?

> 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?

~Andrew

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